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PLOS One logoLink to PLOS One
. 2026 Mar 12;21(3):e0344775. doi: 10.1371/journal.pone.0344775

Factors influencing methamphetamine use among Lahu hill tribe youth in Chiang Rai, Thailand: A case‒control study

Phootawan Thinpanyawong 1,2, Tawatchai Apidechkul 1,3,*, Karl Peltzer 4,5, Pilasinee Wongnuch 1,3
Editor: Ghaffar Ali6
PMCID: PMC12981475  PMID: 41818262

Abstract

Background

Methamphetamine (MA) use presents a significant public health issue that strongly affects human health and social security. Marginalized populations, including youth from the Lahu tribe, which represents the second-largest hill tribe living in the remote border areas of northern Thailand, are among the most vulnerable to MA use. This study aimed to determine the factors associated with MA use among Lahu youths aged 15–24 years who lived in Chiang Rai Province, Thailand.

Methods

A case‒control study was conducted to determine the factors associated with MA use among Lahu youths aged 15–24 years. The Lahu youths who used MA were considered cases of MA use, and those who never used MA were considered controls. The participants were randomly recruited from Mae Suai, Mae Chan, Mae Fah Luang, and Muang Districts, Chiang Rai Province, Thailand, the areas of Thailand most commonly populated by the Lahu people. A validated questionnaire was used to collect data, and multiple logistic regression was used to detect association factors with a statistical significance set at α = 0.05.

Results

A total of 136 cases and 272 controls were included in the study. The majority of the participants were male (79.4%), 77.9% held a Thai national identification card, and 24.0% had completed primary school. After controlling for age, those females were 5.29-fold (95% CI = 2.02–13.89) more likely to use MA than males. Those with easy-to-access MA had a 2.28-fold (95% CI = 1.14–4.54) higher likelihood of MA use than those who had difficulty accessing. Those who had no schooling had a 4.29-fold (95% CI = 1.17–15.73) higher likelihood of MA use than those who had vocational and university education. Those who were unemployed had a 3.92-fold (95% CI = 1.14–13.37) higher likelihood of MA than those who were students. Those who smoked or had ever smoked had a 29.08-fold (95% CI = 11.57–73.12) greater likelihood of MA use than those who never smoked. Those who drank alcohol had a 2.53-fold (95% CI = 1.05–6.11) higher likelihood of MA use than those who did not. Those who had high knowledge scores of MA prevention and control would be more likely to use MA than those who had lower scores (AOR = 1.29; 95% CI = 1.11–1.50).

Conclusions

An effective intervention and policy that focuses on females with low levels of education, unemployed, and having a history of substance use should be urgently developed and implemented to reduce MA use among Lahu youths.

Introduction

Methamphetamine (MA) is a strongly addictive stimulant that affects the central nervous system (CNS) by releasing and inhibiting the reabsorption of serotonin, norepinephrine, and dopamine [1,2]. High levels of these neurotransmitters in synapses lead to euphoria, insomnia, increased libido, and increased energy for a longer time awake [3]. MA abuse is defined as the use of MA in any context, leading to long-term use with both physical and mental health consequences [4–6], including suicidal behaviors [7]. The long-term use of MA can lead to several chronic diseases [8]. MA not only negatively affects the health of users of MA but also affects the family’s finances and the security of people in the community [4]. People initiate MA use for various reasons, such as curiosity [9], having a positive viewpoint of MA use [10–11], and pressure from peers [11] or by family members [11]. Youths are the group most vulnerable to MA use [12].

The United Nations Office on Drugs and Crime (UNODC) reported that in 2019, the majority of amphetamine-type stimulant (ATS) substances were MA, and 271 million people (5.5%) used narcotic drugs worldwide; among these, 29 million used MA, and this number has been increasing each year [13]. The majority of users of MA use began using MA while they were young, typically around the age of 22 years [14]. Previous studies of European populations reported MA use beginning at a median of 20 years of age [15]. More than 120 tons of MA were produced in Southeast Asia in 2018 [16]. Thailand, particularly the northern region of Thailand, has been reported as one of the largest sources of MA worldwide [17]. MA is the most commonly used drug substance across Thailand [18]. Among new registered patients in substance abuse clinics, 26.5% are under 25 years of age [18,19]. High rates of cigarette use and alcohol consumption are also observed within this age group, which could ultimately lead to MA use [20–22].

Chiang Rai Province is located in northernmost Thailand, with a population of more than 1.2 million people, and has been reported as a major part of the route through which MA is imported from neighboring countries, transported to large cities in Thailand, and distributed throughout Thailand or exported to other countries [23]. More than 30% of the people living in Chiang Rai Province are members of hill tribes, each with its own culture, language, and traditional subsistence crops grown for daily survival [23]. There are six main tribes in the Chiang Rai Province: the Akha, Lahu, Hmong, Yao, Karn, and Lisu peoples. After the Akha people, the Lahu population is the second largest of these groups [20]. A large proportion of hill tribe people, including the Lahu people, live below Thailand's national poverty line ($1400 per year per family) [24,25]. Many Lahu people are not granted a Thai identification card, which is required for all public services, including health care services; the national Thai government allocates resources to only those who hold an ID card [26]. Several conditions, including the distance from their village to a school and poor family economics, also limit Lahu youths’ access to education [27]. These conditions promote MA use among the Lahu people as a means of relieving life stressors and participation in MA trade to obtain money for family members, particularly the youths.

Given their poor socioeconomic status and location along the border, Lahu youths have been reported as having one of the highest rates of MA use, with 14.8% of those aged 15–24 years using MA [25]. However, studies on the factors associated with MA use among Lahu youths are lacking. The present study aimed to determine the factors associated with MA use among Lahu youths aged 15–24 years n Chiang Rai Province, Thailand. These findings could be used to generate effective public health policies and interventions to address this particular issue.

Methods

Study design

A community-based case‒control study design was used for data collection to test the hypothesis that education, individual health risk behaviors, and peer relationships are associated with MA use among Lahu youths.

Study population

Lahu youths aged 15–24 years who were living in Lahu villages in Chiang Rai Province, Thailand, comprised the study population.

Sample size

The sample size was calculated based on information obtained from a previous report about the MA use in this population in Thailand [19]. The sample size was calculated using the standard formula for a case‒control study [28] and on the basis of the following assumptions: p1 (proportion of exposed cases based on previous study) = 0.884 [32], p2 (proportion of exposed controls based on previous study) = 0.769 [32], q1 – (1 – p1) = 0.116, q2 – (1 – p2) = 0.231, Z1‒α(confidence at an error of 5.0%)= 1.96, Z1‒β(the power of the study at 80.0%) = 0.84, r (the ratio of cases to controls) = 2, and the odds ratio (OR) from the previous study = 2.29 [19]. Using these assumptions, we calculated that a total of 136 cases and 272 controls (a control/case ratio of 2.0) were needed for the analysis.

A total of 136 cases were randomly selected from the 227 cases of MA use in the clinics between January 2022 and March 2024. No individuals refused to provide this information. However, three selected cases moved to other provinces and could not be reached for information. In addition, 276 gender-matched controls were selected from the villages.

Study sample

The study participants were recruited from Lahu youths who lived in Mae Fah Luang, Mae Chan, Mae Suai, and Muang Districts, Chiang Rai Province. The cases included Lahu youth who currently use or have previously used MA. The inclusion criteria for cases included (i) members of the Lahu tribe aged 15–24 years, (ii) self-reported MA use, and (iii) participation in the MA treatment program as a client. The controls were Lahu youth who had never used MA, and the inclusion criteria for controls included (i) members of the Lahu tribe between 15 and 24 years old and (ii) triple-negative MA testing results based on the following: (a) never-user of MA by self-report, (b) a negative MA urine test result, and (c) a non-MA user consensus in the triple-check method. However, patients and controls with cognitive impairment and those who were unable to communicate were excluded from the study. Individuals who had a positive result in one of the three methods were excluded from the control group.

A triple-check method was used to confirm the MA use status of the control by identifying three people who knew the potential participants (a village headman, a close friend, and a family member). These individuals were asked whether the potential participant used MA or not. The responses were “yes” or “no”. If any of these persons responded “yes”, the potential participant was excluded from the control selection. The information obtained remained with the researcher without being shared with anyone. The method aimed to ensure that all selected controls had never used MA.

Recruitment procedure

Lahu youths living in Lahu village in Chiang Rai Province who attended any of four district hospital MA clinics, namely, the Mae Fah Luang, Mae Chan, Muang, and Mae Suai district hospitals, between January 2022 and March 2024 were listed and selected randomly as cases. Once a case was selected, the village headman was contacted to obtain a list of Lahu youth aged 15–24 years who did not use MA living in the village. At the meeting with the village headman to learn where the case(s) lived, information about the project was provided. Youths aged 15–24 years who did not use MA and lived in the same village as the cases were randomly selected as controls. Controls were matched by sex to cases.

A triple-check method was applied to confirm the lack of MA use among the control. Once two controls for each case were obtained, the potential controls were asked for a voluntary urine test. The participants were asked to complete a self-administered questionnaire. Identifiable personal information was not included in the questionnaire. During the completion of the questionnaire, all the participants were informed that they were encouraged to ask any questions if they did not understand parts of the questionnaire. All parts of the questionnaire and testing were conducted in a private room after the participants agreed to participate in the study. This process continued until two controls were enrolled for each case from a given village. If we were unable to find controls in the same village as the case, a control was selected from another nearby Lahu village. All steps of the recruitment process were performed voluntarily and privately using a confidential and anonymous system. The participants were informed at the beginning of data collection that they were able to withdraw from the study at any time. Data were collected between June 2023 and March 2024.

Research instruments

A validated questionnaire was used as the primary research instrument and was developed from a literature review [1,8,11,25] and discussion with experts and local people. Two nurses who were working in the MA cessation clinic in the regional hospital located in Chiang Rai Province and four local people (two Lahu village headmen and two Lahu village health volunteers) discussed the situations and factors related to MA use among the Lahu youths. The discussion with two nurses started with a general question about the clinic and then focused on the specific questions about the Lahu youths who attended the clinics, particularly the risk behaviors leading to MA use. Discussions with local people, both the village headman and the village’s health volunteer, were conducted in two villages. The main discussion focused on the MA use among young people and the factors influencing MA use. All the preliminary information obtained from these two forums was included for questionnaire development.

Finally, the questionnaire consisted of three parts. In part one, 13 questions were used to collect demographic information, including social media experience. For example, the questions in this section included a question asking participants about gender with three options for their response—male, female, or other—and a question asking about social media use with six options: Facebook, Line, Twitter (X), Instagram, or other. Participants checked the appropriate box for any social media platform that they used and left blank those they did not use. Regarding the income, a 15,000-baht monthly salary was used as the cutoff according to the average recent graduate (typically aged 22–24 years) in Thailand. In part two, ten questions were used to collect information on substance use behaviors, including questions such as “Do you smoke?”, with three response options: “yes”, “ever”, and “no”. When the participants responded “yes”, they were asked about their frequency of use and the availability of cigarettes. Part three was divided into two subsections: ten questions were used to detect knowledge of MA prevention and control, and another ten questions were used to detect attitudes toward MA prevention and control. In the knowledge section, each question had three options: “correct”, “incorrect”, and “unknown”. For positive questions, anyone who responded “correct” was given a score of one, and those who responded “incorrect” or “unknown” were given a score of 0. On the other hand, for the negative question, those who responded “incorrect” were given a value of “1”, whereas those who responded “correct” and “unknown” were given a value of “0”. There were five positive and five negative questions in the knowledge section. At the end of the questionnaire, those scoring ≤ 4 were defined as having poor knowledge, those scoring 5–7 points were defined as having moderate knowledge, and those scoring 8 and above were defined as having high knowledge [29,30]. With respect to attitudes, there were ten questions with three options for each question: “agree”, “neutral”, and “disagree”. There were five positive questions and five negative questions. Those who answered “agree,” “neutral,” and “disagree” with a positive question were given 2, 1, and 0, respectively. For the negative questions, those who answered “agree,” “neutral,” or “disagree” with a positive question were given 0, 1, or 2, respectively. The total score was 20. At the end of the questionnaire, those who had scores between 0 and 6 were considered to have a negative attitude toward MA use, those with scores between 7 and 13 were considered to have a neutral attitude, and those with a score of 14 or greater were considered to have a positive attitude [29–31]. In the last part, 24 questions were used to collect information about the subjects’ social environment related to MA use among the Lahu youth, such as having had peers who used MA and having had a family member who used MA (Supplement File 1).

The questionnaire was validated by the item-objective congruent (IOC) method using three external experts’ opinions on each question: one professional psychologist and one nurse who were working at the MA cessation clinic and one healthcare provider working in a health-promoting hospital. Each expert provided a score for each item with three choices: “-1” for questions not relevant to the study, “0” for questions relevant to the study but requiring improvement, and “+1” for questions completely relevant to the study. Once the scores from three experts on each item were obtained, the average score was calculated and interpreted. If the average score was less than 0.5, the question was deleted from the questionnaire. If the average score was between 0.5 and 0.7, the question was revised as suggested by the experts before they were considered further. Those questions that scored over 0.7 could be further revised or left unaltered in the final form. Afterward, the questionnaire was tested in a pilot study in Mae Chan District, Chiang Rai Province, Thailand, on 20 individuals whose characteristics were similar to those of the study population. During the pilot, seven (11) questions/points were found that required improvement before use, such as a question asking about the education of the participants, which was confusing because some people did not complete three years of secondary school and instead dropped out of school after one or one and a half years of secondary school. In this case, the question was modified to as about the participants’ completed level of their education with six (6) choices: no education, primary school, secondary school, high school, vocational school, and a bachelor's degree or above. In addition, the reliability of the ten (10) knowledge questions and the ten (10) attitude questions was evaluated using a Cronbach’s alpha of 0.78 and 0.83, respectively. Bioline methamphetamine strips were used to detect MAs in urine; these strips have a sensitivity of 98.0% and a specificity of 99.0% (Pacific Biotech Co., Ltd., Bangkok, Thailand) [32].

Statistical analysis

The data were analyzed using SPSS (Version 24; Chicago, IL). Missing data were checked and managed properly before further analysis. Missing data were excluded from all subsequent analyses and reporting in this study. Descriptive statistics were used to present the general characteristics of the participants. Categorical data are presented as percentages, whereas continuous data are presented as the means and standard deviations. Chi-square tests and multiple logistic regression were used to detect factors associated with MA use at a significance level of α = 0.05. Collinearity was tested to determine high correlations among the variables before they were entered into the logistic regression. In this step, cannabis accessibility and opium accessibility were found to have a correlation of r = 0.87; thus, we maintained only opium accessibility in the final prediction model. The variance inflation factor (VIF) and tolerance were also tested with all the variables included in the model. No variables with a VIF > 5 and a tolerance < 0.2 were detected. Univariable logistic regression was used to explore the associations between independent variables and MA use at a p-value = 0.05. Stepwise multiple logistic regression with a probability of 0.2 was used to maintain or exclude variables from the model before fitting the final model. At the end of the analysis, age in continuous form was controlled for in the model because it was found to be a significant confounding factor in previous studies [33]. The pseudo-R-squared values and Hosmer–Lemeshow test were used to examine the model's fit and for interpretation.

Ethical approval and consent to participate

The proposal and relevant protocols were approved by the Chiang Rai Provincial Health Office Ethical Committee, No. CROOHP 39/2566. Before the interviews, the essential information was clearly communicated in Thai and local languages to the participants. Informed consent was obtained voluntarily. The interviews were conducted in private and confidential rooms. For Lahu youth aged less than 18 years, parents were asked to provide permission for their personal information by signing an informed consent form voluntarily. The study procedures were performed in accordance with the relevant guidelines and regulations and within the Declaration of Helsinki of 1975, as revised in 2000 (5).

Results

Four hundred and eight participants were recruited and analyzed in the study: 79.4% were male, 77.9% held a Thai national identification card, 24.0% graduated from primary school, and 34.8% had greater than a high school education. More than half (59.1%) had one partner, and 75.9% were unmarried. The parents of the cases lived together 18% less often than those of the controls did (p-value <0.001). Half of them (50.5%) had moderate knowledge of MA prevention and control, and 63.2% had moderate attitudes toward MA prevention and control (Table 1).

Table 1. Comparisons of general characteristics between cases and controls.

General characteristics Total Cases Controls χ2 p-value
n
% n
% n
%
Total 408 100.0 136 33.3 272 66.7 N/A N/A
Age (years)
15-19 243 59.6 76 55.9 167 61.4 1.14 0.285
20-24 165 40.4 60 44.1 105 38.6
Mean = 19.11, SD = 2.84
Gender
Male 324 79.4 108 79.4 216 79.4 0.00 1.000
Female 84 20.6 28 20.6 56 20.6
Identification card
Thai national card 318 77.9 100 73.5 218 80.2 2.03 0.163
Non-Thai national card/None 90 22.1 36 26.5 54 19.9
Education
Primary school 98 24.0 44 32.4 54 19.9 41.49 <0.001*
Secondary school 100 24.5 34 25.0 66 24.3
High school/Vocational/University 142 34.8 21 15.4 121 44.5
No schooling 68 16.7 37 27.2 31 11.4
Occupation
Private or government employee/Trader 40 9.8 7 5.1 33 12.1 45.70 <0.001*
Agriculturist 47 11.5 23 16.9 24 8.8
Daily wage 117 28.7 47 34.6 70 25.7
Student 123 30.1 17 12.5 106 39.0
Unemployed 81 19.9 42 30.9 39 14.3
Household income (baht/month) (n = 335)
< 15000 293 87.5 105 95.5 188 83.6 9.54 0.002*
≥ 15001 42 12.5 5 4.5 37 16.4
Number of partners (persons)
One 241 59.1 81 59.6 160 58.8 5.34 0.072
≥ 2 53 13.0 24 17.6 29 10.7
No 114 27.9 31 22.8 83 30.5
Marital status
Single 310 76.0 103 75.7 207 76.1 0.20 0.922
Married 88 21.6 29 21.3 59 21.7
Separated/Divorced/Widowed 10 2.5 4 2.9 6 2.2
Parents’ living status
Living together 292 71.6 81 59.6 211 77.6 14.57 0.001*
Separated 53 13.0 26 19.1 27 9.9
Divorced/either one or both died 63 15.4 29 21.3 34 12.5
Knowledge toward MA prevention and control
High 123 30.1 48 35.3 75 27.6 21.23 <0.001*
Moderate 206 50.5 79 58.1 127 46.7
Low 79 19.4 9 6.6 70 25.7
Attitude toward MA prevention and control
Good 103 25.2 17 12.5 86 31.6 20.01 <0.001*
Moderate 258 63.2 96 70.6 162 59.6
Poor 47 11.5 23 16.9 24 8.8

*Significant at a p-value≤ of 0.05

Several characteristics were significantly different between the case and control groups: education, occupation, household income, parental living status, knowledge of MA prevention and control, and attitudes toward MA prevention and control (Table 1).

With respect to substance use behaviors, as shown in Table 2, 55.1% had consumed alcohol, and 43.9% had smoked. The majority (90.9%) had low stress, and friends (59.6%) were the people whom they first consulted with while in need, followed by family members and partners. Several behaviors differed between those who used MA and those who did not. Compared with the controls, those with MA use were more likely to smoke cigarettes and drink alcohol and more frequently consumed whisky, beer, cannabis, opium, and heroin (Table 2).

Table 2. Comparisons of risk behaviors for MA use between the case and control groups.

Substances use behaviors Total Cases Controls χ2 p-value
n
% n
% n
%
Cigarette smoking
Ever 179 43.9 116 85.3 59 21.7 149.73 <0.001*
No 229 56.1 20 14.7 213 78.3
Current smoker and amount per day (n = 116)
Current smoker 116 100.0 92 79.3 24 20.7
≤ 10 Cigarettes/day 60 51.7 45 48.9 15 62.5 1.41 0.236
≥ 11 Cigarettes/day 56 48.3 47 51.1 9 37.5
Alcohol use
Ever 225 55.1 117 86.0 108 39.7 78.65 <0.001*
No 183 44.9 19 14.0 164 60.3
Frequency of whisky consumption among current users (n = 130)
Current drinking 130 100.0 82 63.1 48 36.9
Once/month 47 36.2 17 20.7 30 62.5 23.15 <0.001*
2-4 times/month 60 46.2 48 58.5 12 23.5
2-3 times/week 23 17.7 17 20.7 6 12.5
Frequency of beer consumption among current users (n = 134)
Current drinking 134 100.0 79 59.0 55 41.0
Once/month 35 26.1 14 17.7 21 38.2 9.28 0.010*
2-4 times/month 79 59.0 51 64.6 28 50.9
2-3 times/week 20 14.9 14 17.7 6 10.9
Cannabis use
Ever 90 22.1 59 43.4 30 11.0 55.64 <0.001*
No 318 77.9 77 56.6 242 89.0
Frequency of cannabis use among current users of cannabis (n = 36)
Current use 36 100.0 21 58.3 15 41.7
≤ 1 times/week 23 63.9 12 57.1 11 73.3 0.99 0.319
≥ 2 times/week 13 36.1 9 42.9 4 26.7
Opium use
Ever 45 11.0 36 26.5 9 3.3 49.56 <0.001*
No 363 89.0 100 73.5 263 96.7
Frequency of opium use among current users of opium (n = 19)
Current use 19 100.0 14 73.6 5 26.4
1-2 times/month 11 57.9 7 50.0 4 80.0 3.35 0.181a
≥ 2 times/week 8 52.1 7 50.0 1 20.0
Heroin
Ever 47 11.5 41 30.1 6 2.2 69.44 <0.001*
No 361 88.5 95 69.9 266 97.8
Frequency of heroin use among current users of heroin (n = 23)
Currently use 23 100.0 18 78.3 5 21.7
≤ 1 times/week 5 21.7 4 22.2 1 20.0 0.01 1.000a
≥ 2 times/week 18 78.3 14 77.8 4 80.0
Stress (ST-5)
Moderate to very high 37 9.1 10 7.4 27 9.9 0.72 0.393
No 371 90.9 126 92.6 245 90.1
Social support during a personal challenge
Friends
Yes 243 59.6 79 58.1 164 60.3 0.18 0.669
No 165 40.4 57 41.9 108 39.7
Partners
Yes 211 51.7 76 55.9 135 49.6 1.42 0.234
No 197 48.3 60 44.1 137 50.4
Family member
Yes 219 53.7 49 36.0 170 62.5 25.55 <0.001*
No 189 46.3 87 64.0 102 37.5
Social media use
Facebook
Yes 319 78.2 106 77.9 213 78.3 0.01 0.932
No 89 21.8 30 22.1 59 21.7
Line
Yes 285 69.9 97 71.3 188 69.1 0.21 0.647
No 123 30.1 39 28.7 84 30.9
Twitter (X)
Yes 55 13.5 4 2.9 51 18.9 19.43 <0.001*
No 353 86.5 132 97.1 221 81.2
Instagram
Yes 57 14.0 7 5.1 50 18.4 13.22 <0.001*
No 351 86.0 129 94.9 222 81.6

* Significant at p-value≤ 0.05; a = Fisher’s exact test

Facebook (78.2%) and Line (69.9%) were the most commonly used social media platforms, but use levels were not significantly different between cases and controls. Sharing personal challenges with family members differed significantly between groups, with only 36.0% of the cases doing so compared to 62.5% of the controls (p-value<0.001)—cases were much more isolated from family with respect to communication. While approximately 1 in 5 controls used Twitter (X) and Instagram, fewer cases used both (2.9% and 5.1%), both with p-values <0.001 (Table 2).

In Table 3, half of the participants (52.9%) reported having friends used alcohol, and 49.0% reported having friends smoked. A total of 62.7% reported having a family member who used alcohol, and 59.3% had a family member who smoked. Moreover, the number participants reporting peer pressure from friends to try MA (27.2%) was greater than the number reporting pressure from family members (3.7%). Most of the participants reported that they did not find it difficult to access cigarettes (98.8%) or alcoholic beverages (99.8%).

Table 3. Comparison of the impact of proximity and accessibility to substances between the case and control groups.

Substance use Total Cases Controls χ2 p-value
n
% n
% n
%
Having a friend who smoked
Yes 200 49.0 101 74.3 99 36.4 52.03 <0.001*
No/Unsure 208 51.0 35 25.7 173 63.6
Having a friend who drank alcohol
Yes 216 52.9 97 71.3 119 43.8 27.67 <0.001*
No/Unsure 192 47.1 39 28.7 153 56.2
Having a friend who used cannabis, opium, or heroin
Yes 131 32.1 75 55.1 56 20.6 49.68 <0.001*
No/Unsure 277 67.9 61 44.9 216 79.4
Having a friend who used MA
Yes 107 26.2 78 57.4 29 10.7 102.16 <0.001*
No/Unsure 301 73.8 58 42.6 243 89.3
Having been pressured by a friend to use MA
Yes 111 27.2 85 62.5 26 9.6 128.31 <0.001*
No 297 72.8 51 37.5 246 90.4
Having a family member who smoked
Yes 242 59.3 101 74.3 141 51.8 18.90 <0.001*
No 166 40.7 35 25.7 131 48.2
Having a family member who used alcohol
Yes 256 62.7 107 78.7 149 54.8 22.15 <0.001*
No 152 37.3 29 21.3 123 45.2
Having a family member who used cannabis, opium, or heroin
Yes 44 10.8 21 15.4 23 8.5 4.60 0.041*
No 364 89.2 115 84.6 249 91.5
Having a family member who used MA
Yes 37 9.1 20 14.7 17 6.2 7.86 0.005*
No 371 90.9 116 85.3 255 93.8
Having been pressured by a family member to use MA
Yes 15 3.7 10 7.4 5 1.8 7.79 0.005*
No 393 69.3 126 92.6 267 98.2
Cigarette accessibility
Easy-to-moderate 403 98.8 135 99.3 268 98.5 0.41 0.669a
Difficulty 5 1.2 1 0.7 4 1.5
Alcohol accessibility
Easy-to-moderate 407 99.8 136 100.0 271 99.6 0.50 1.000a
Difficulty 1 0.2 0 0.00 1 0.4
Cannabis accessibility
Easy-to-moderate 111 27.2 59 43.4 52 19.1 26.96 <0.001*
Difficulty 297 72.8 77 56.6 220 80.9
Opium accessibility
Easy-to-moderate 52 12.7 33 24.3 19 7.0 24.34 <0.001*
Difficulty 356 87.3 103 75.7 253 93.0
Heroin accessibility
Easy-to-moderate 71 17.4 37 27.2 34 12.5 13.64 <0.001*
Difficulty 337 82.6 99 72.8 238 87.5
MA accessibility
Easy 256 62.7 106 77.9 150 55.1 20.15 <0.001*
Difficulty 152 37.3 30 22.1 122 44.9

* Significant at a p-value≤ of 0.05

Many experiences were significantly different between those with MA use and those without MA use, with a greater proportion of the following among cases: having a friend who smoked; having a friend who drank alcohol; having a friend who used cannabis, opium, or heroin; having a friend who used MA; having been pressured by a friend to use MA; having a family member who smoked; having a family member who drank alcohol; having a family who used cannabis, opium, or heroin; having a family who used MA; having been pressured by a family member to use MA; and having access to MA. Compared with the controls, the cases had much greater access to cannabis, opium, and heroin (all p-values <0.001).

The study included several independent variables or predictors, including age, sex, marital status, education, occupation, family income, knowledge of MA prevention and control, and attitudes toward MA prevention and control. The dependent variable or outcome was MA use. Age was treated as a confounder in the model.

Several variables were found to be associated with MA use among Lahu youths in the univariate logistic regression model: education; occupation; household income; knowledge of MA prevention and control; attitudes toward MA prevention and control; smoking status; alcohol consumption; cannabis use; opium use; heroin use; parental living status; having friends who smoke cigarettes; having friends who use cannabis, opium, or heroin; having friends who use MA; having been pressured by friends to use MA; having family members who smoke cigarettes; having family members who drink alcohol; having family members who use cannabis, opium, or heroin; having family members who use MA; having been pressured by family members to use MA; cannabis accessibility; opium accessibility; heroin accessibility; and MA accessibility (Table 4).

Table 4. Factors associated with MA use among Lahu youth by univariate analysis.

Factors Cases Controls OR 95% CI p-value
Age (years)
15-19 76 (55.9) 167 (61.4) 1.00
20-24 60 (44.1) 105 (38.6) 1.26 0.82-1.91 0.285
Gender
Male 108 (79.4) 216 (79.4) 1.00
Female 28 (20.6) 56 (20.6) 1.00 0.60-1.66 1.000
Identification card
Thai national card 100 (73.5) 218 (80.1) 1.00
Non-Thai national card/None 36 (26.5) 54 (19.9) 1.45 0.87-2.36 0.130
Number of partners (person)
One 81 (59.6) 160 (58.8) 1.36 0.83-2.22 0.225
≥ 2 24 (17.6) 29 (10.7) 2.22 1.12-4.38 0.022*
No 31 (22.8) 83 (30.5) 1.00
Married status
Single 103 (75.7) 207 (76.1) 1.00
Married 29 (21.3) 59 (21.7) 0.97 0.58-1.63 0.962
Separated/Divorce/Widowed 4 (2.9) 6 (2.2) 1.34 0.37-4.85 0.656
Parental living status
Living together 81 (59.6) 211 (77.6) 1.00
Separated 26 (19.1) 27 (9.9) 2.51 1.38-4.55 0.003*
Divorced/either one or both died 29 (21.3) 34 (12.5) 2.22 1.27-3.88 0.005*
Education
High School/Vocational/University 21 (15.4) 121 (44.5) 1.00
Secondary school 34 (25.0) 66 (24.3) 2.97 1.60-5.52 0.001*
Primary school 44 (32.4) 54 (19.9) 4.70 2.55-8.65 <0.001*
No schooling 37 (27.2) 31 (11.4) 6.88 3.54-13.37 <0.001*
Occupation
Private/Governance employee/Trader 7 (5.1) 33 (12.1) 1.32 0.51-3.47 0.569
Agriculturist 23 (16.9) 24 (8.8) 5.98 2.77-12.88 <0.001*
Daily wage worker 47 (34.6) 70 (25.7) 4.19 2.27-7.87 <0.001*
Unemployed 42 (30.9) 39 (14.3) 6.72 3.43-13.16 <0.001*
Student 17 (12.5) 106 (39.0) 1.00
Knowledge of MA prevention and control
High 48 (35.3) 75 (27.6) 4.98 2.27-10.89 <0.001*
Moderate 79 (58.1) 127 (46.7) 4.84 2.28-10.22 <0.001*
Low 9 (6.6) 70 (25.7) 1.00
Attitude toward MA prevention and control
Good 17 (12.5) 86 (31.6) 1.00
Moderate 96 (70.6) 162 (59.6) 3.00 1.68-5.34 <0.001*
Poor 23 (16.9) 24 (8.8) 4.85 2.23-10.50 <0.001*
Stress status
Moderate to very high 10 (7.4) 27 (9.9) 1.00
No 126 (92.6) 245 (90.1) 1.39 0.65-2.96 0.395
Social support during a personal challenge
Friends
Yes 79 (58.1) 164 (60.3) 1.00
No 57 (41.9) 108 (39.7) 1.10 0.72-1.67 0.669
Partners
Yes 76 (55.9) 135 (49.6) 1.00
No 60 (44.1) 137 (50.4) 0.78 0.51-1.18 0.234
Family member
Yes 49 (36.0) 170 (62.5) 1.00
No 87 (64.0) 102 (37.5) 3.76 2.30-6.21 <0.001*
Social media use
Facebook
Yes 106 (77.9) 213 (78.3) 1.00
No 30 (22.1) 59 (21.7) 1.02 0.62-1.68 0.932
Line
Yes 97 (71.3) 188 (69.1) 1.00
No 39 (28.7) 84 (30.9) 0.90 0.57-1.14 0.647
Twitter
Yes 4 (2.9) 51 (18.8) 1.00
No 132 (97.1) 221 (81.2) 7.62 2.69-21.55 <0.001*
Instagram
Yes 7 (5.1) 50 (18.4) 1.00
No 129 (94.9) 222 (81.6) 4.15 1.83-9.43 <0.001*
Cigarette smoking
Yes/Ever 120 (88.2) 59 (21.7) 27.10 14.91-49.14 <0.001*
No 16 (11.8) 213 (78.3) 1.00
Alcohol consumption
Yes/Ever 117 (86.0) 108 (39.7) 9.35 5.43-16.08 <0.001*
No 19 (14.0) 164 (60.3) 1.00
Cannabis use
Yes/Ever 60 (44.1) 30 (89.0) 6.37 3.83-10.58 <0.001*
No 76 (55.9) 242 (11.0) 1.00
Opium use
Yes/Ever 36 (80.0) 9 (20.0) 10.52 4.89-22.62 <0.001*
No 100 (27.5) 263 (72.5) 1.00
Heroin use
Yes/Ever 41 (30.1) 6 (2.2) 19.13 7.87-46.50 <0.001*
No 95 (69.9) 266 (97.8) 1.00
Having friend who smokes cigarette
Yes 101 (74.3) 99 (36.4) 5.04 3.19-7.86 <0.001*
No/Unsure 35 (25.7) 173 (63.6) 1.00
Having friend who drinks alcohol
Yes 97 (71.3) 119 (43.8) 3.20 2.05-4.97 <0.001*
No/Unsure 39 (28.7) 153 (56.2) 1.00
Having friend who used cannabis, opium, or heroin
Yes 75 (55.1) 56 (20.6) 4.74 3.03-7.42 <0.001*
No/Unsure 61 (44.9) 216 (79.4) 1.00
Having a friend who used MA
Yes 78 (57.4) 29 (10.7) 11.26 6.74-18.83 <0.001*
No 58 (42.6) 243 (89.3) 1.00
Having been pressured by a friend to use MA
Yes 85 (62.5) 26 (9.6) 15.77 9.25-26.86 <0.001*
No 51 (37.5) 246 (90.4) 1.00
Having a family member who smokes cigarette
Yes 101 (74.3) 141 (51.8) 2.68 1.70-4.21 <0.001*
No 35 (25.7) 131 (48.2) 1.00
Having a family member who drinks alcohol
Yes 107 (78.7) 149 (54.8) 3.05 1.89-4.89 <0.001*
No 29 (21.3) 123 (45.2) 1.00
Having a family member who uses cannabis, opium, or heroin
Yes 21 (15.4) 23 (8.5) 1.98 1.05-3.72 0.034*
No 115 (84.6) 249 (91.5) 1.00
Having a family member who uses MA
Yes 20 (14.7) 17 (6.2) 2.59 1.30-5.11 0.006*
No 116 (85.3) 255 (93.8) 1.00
Having been pressured by a family member to use MA
Yes 10 (7.4) 5 (1.8) 4.24 1.41-12.65 0.010*
No 126 (92.6) 267 (98.2) 1.00
Cigarette accessibility
Easy-to-moderate 135 (99.3) 268 (98.5) 2.02 0.22-18.20 0.533
Difficulty 1 (0.7) 4 (1.5) 1.00
Alcohol accessibility
Easy-to-moderate 136 (100.0) 271 (99.6) N/A N/A N/A
Difficulty 0 (0.00) 1 (0.4) 1.00
Cannabis accessibility
Easy-to-moderate 59 (43.4) 52 (19.1) 3.24 2.06-5.11 <0.001*
Difficulty 77 (56.6) 220 (80.9) 1.00
Opium accessibility
Easy-to-moderate 33 (24.3) 19 (7.0) 4.27 2.32-7.85 <0.001*
Difficulty 103 (75.7) 253 (93.0) 1.00
Heroin accessibility
Easy-to-moderate 37 (27.2) 34 (12.5) 2.62 1.55-4.41 <0.001*
Difficulty 99 (72.8) 238 (87.5) 1.00
MA accessibility
Easy 106 (77.9) 150 (55.1) 2.87 1.79-46.00 <0.001*
Difficulty 30 (22.1) 122 (44.9) 1.00

* Significant at a p-value≤ of 0.05

In the univariate analysis (Table 4) it is notable that the influence of friends exceeded that of the influence of family in the odds ratios in each category regarding Smoking, Alcohol use, Cannabis use, MA use, and pressure to use MA – in some cases, the Friends ORs were more than two- to three-fold higher than Family ORs. This has implications for prevention and intervention concerning social networks.”

After controlling for age in the stepwise multivariate logistic regression, ten (10) variables remained in the final prediction model (Table 5). Among these variables, seven (7) variables were found to be associated with MA use among Lahu youths. Those females were 5.29-fold (95% CI = 2.02–13.89) more likely to use MA than males. Those with easy-to-access MA had a 2.28-fold (95% CI = 1.14–4.54) higher likelihood of MA use than those who had difficulty accessing. Those who had no schooling had a 4.29-fold (95% CI = 1.17–15.73) higher likelihood of MA use than those who had vocational and university education. Those who were unemployed had a 3.92-fold (95% CI = 1.14–13.37) higher likelihood of MA than those who were students. Those who smoked or had ever smoked had a 29.08-fold (95% CI = 11.57–73.12) greater likelihood of MA use than those who never smoked. Those who drank alcohol had a 2.53-fold (95% CI = 1.05–6.11) higher likelihood of MA use than those who had never drunk. Those who had high knowledge scores of MA prevention and control would be more likely to use MA than those who had lower scores (AOR = 1.29; 95% CI = 1.11–1.50).

Table 5. Factors associated with MA use among Lahu youth by multivariate logistic regression.

Factors AOR 95% CI p-value
Sex
Male 1.00
Female 5.27 1.98-13.82 <0.001*
MA accessibility
Easy 2.35 1.18-4.65 0.015*
Difficulty 1.00
Married status
Single 1.00
Married 0.76 0.31-1.84 0.543
Separated/Divorce/Widowed 0.48 0.75-3.02 0.431
Parental living status
Living together 1.00
Separated 1.73 0.70-4.31 0.236
Divorced/either one or both died 1.70 0.70-4.10 0.239
Education
High School/Vocational/University 1.00
Secondary school 0.86 0.29-2.57 0.783
Primary school 1.46 0.42-5.02 0.552
No schooling 4.07 1.12-14.75 0.033*
Occupation
Private/Governance employee/Trader 1.53 0.34-6.87 0.578
Agriculturist 2.45 0.52-11.49 0.256
Daily wage worker 2.85 0.79-10.24 0.108
Unemployed 3.75 1.10-12.76 0.034*
Student 1.00
Knowledge of MA prevention and control (Score) 1.30 1.12-1.51 <0.001*
Cigarette smoking
Yes/Ever 27.08 11.07-66.22 <0.001*
No 1.00
Alcohol consumption
Yes/Ever 2.68 1.12-6.38 0.026*
No 1.00
Sex
Male 1.00
Female 5.29 2.02-13.89 0.001*
Cigarette accessibility
Easy-to-moderate 7.65 0.25-229.55 0.241
Difficulty 1.00
MA accessibility
Easy 2.28 1.14-4.54 0.019*
Difficulty 1.00
Married status
Single 1.00
Married 0.80 0.33-1.97 0.640
Separated/Divorce/Widowed 0.48 0.07-3.07 0.433
Parental living status
Living together 1.00
Separated 1.71 0.68-4.27 0.246
Divorced/either one or both died 1.85 0.75-4.54 0.176
Education
High School/Vocational/University 1.00
Secondary school 0.84 0.28-2.53 0.762
Primary school 1.46 0.42-5.06 0.543
No schooling 4.29 1.17-15.73 0.028*
Occupation
Private/Governance employee/Trader 1.56 0.34-7.17 0.567
Agriculturist 2.24 0.47-10.69 0.309
Daily wage worker 2.81 0.78-10.15 0.114
Unemployed 3.92 1.14-13.37 0.029*
Student 1.00
Knowledge of MA prevention and control (Score) 1.29 1.11-1.50 0.001*
Cigarette smoking
Yes/Ever 29.08 11.57-73.12 <0.001*
No 1.00
Alcohol consumption
Yes/Ever 2.53 1.05-6.11 0.038*
No 1.00

* p-value≤ = 0.05; age was controlled in continuous data form in the analysis

Discussion

Multiple factors contribute to the risk of MA use among Lahu youths, including sex, MA accessibility, low education, unemployed status, knowledge of MA prevention and control, smoking, and drinking alcohol.

Lahu females had a significantly greater likelihood of MA use than males. A study reported that females had a greater number of MA use compared to males [34]. Females were reported to have a greater prevalence of MA use in their lifetime than males in Thailand, even though males had earlier use of MA than females [35]. A study conducted in the United States [36] reported that females had a higher prevalence and frequency of use than males. Weight reduction was the main reason to use MA among females [37]. However, among the Lahu youths, particularly females, the main reason to use MA could be to increase their farming products which is as after using MA, people could spend more time working their farm [11,38].

Lahu youths with low education levels had a significantly greater likelihood of MA use than those with high education levels did. Several studies [12,39] have reported that young people with low education levels have a greater likelihood of engaging in MA use than did those with high education levels do. Another study reported that young people who did not attend school spent time with their friends and eventually used MA [10]. A study conducted in the United States reported that there was a strong association between MA use and low socioeconomic status [40]. A study conducted among the Akha and Lahu people in northern Thailand revealed that those who failed their studies and dropped out of school had a greater likelihood of initiating MA than did those who attended school did [11,25]. Among Lahu youths, those who have a low level of education and who drop out of school have a greater likelihood of using MA because their health literacy is low and because they are more easily pressured by their peers while living together outside the school. Most Lahu villages are located in border and remote areas where access to MA in their daily life is necessary for engaging in MA use.

In our study, it was found that the unemployed status played a role in influencing MA use among the Lahu youths. A study conducted in Iran indicated that unemployed individuals were at risk of MA use than other groups [41], and those who were unemployed individuals were more at risk of relapsing to use substances, including MA [42]. Several studies confirmed that the unemployed working status had a greater risk of MA use than the full-time working status [43,44]. In Thailand, it was found that unemployed young adults were at a higher risk of using MA than employed individuals. The Lahu youths who were not working could spend their time with their friend and be persuaded to use MA. In addition, those who were unemployed could be stressor from having no job and income, which could motivate them to use MA.

Seemingly paradoxically, those who had higher knowledge of MA prevention and control had a significantly greater likelihood of MA use than those at low levels did. A study from Bari Imam [45] reported that nearly half of the participants continued to use their current drug despite knowing the adverse effects. Another study revealed that those with MA use had high knowledge levels of the especially harmful effects of cigarettes, alcohol, and cannabis [46]. For our study, cases were recruited from a list of MA treatment clinics; it is highly possible that those cases in the study received education regarding MA use from health professionals. By contrast, the controls were selected from community members and therefore had low-level knowledge of MA prevention and control. MA users not attending such clinics may also have a much lower level of knowledge. Although a knowledge behavior gap between cases and controls was observed, this study did not focus on this point. In addition, assessing knowledge and MA use could be impacted by the temporal sequence in this study because both knowledge and MA use are assessed simultaneously.

Cigarette smoking and alcohol consumption were associated with a greater likelihood of MA use among Lahu youths than among other groups. Several studies [25,47] have reported a higher rate of MA use among hill tribe people in Thailand who smoke than among those who do not smoke. A systematic review [48] and a study conducted in South Africa [49] confirmed the increased rate of MA use among those who engaged in smoking behavior among young adults. A study conducted in Iran reported that smoking tobacco before MA use was common [2]. Alcohol consumption has been reported as a risk behavior to MA use in several studies [11,43]. A study conducted in Vietnam confirmed the strong association between alcohol use and MA use [50]. Several studies also reported that alcohol use was a common pattern that precedes methamphetamine use [2,51]. Among Lahu youths, cigarette smoking and alcohol were very common because of the accessibility in the villages, and youths may have started using these kinds of substances before starting MA. For these reasons, drug use transition and polydrug use behavior might occur among Lahu youths.

MA accessibility was associated with MA use among Lahu youths. Like other studies in Thailand, accessibility led to individuals initiating MA use [9]. Moreover, the area of shared borders has increased, and MA access near borders is more likely [11]. According to our study area, Lahu youth in remote areas share borders with significant MA smuggling areas [15] with MA available at extremely affordable prices [18], which increases access to MA among Lahu youths.

Limitations

In accordance with the research methodology used in this case‒control study, three approaches were used to identify controls: verbal questions, a urine test, and three references from community members. Although these were the best methods for identifying the controls, they did not guarantee that all the controls had never used MA. An advantage of a case‒control study is that it can assess the causal relationship between variables using the odds ratio, an indirect method. In this study, cases were randomly selected from clinic lists, which may not be a good representation of all MA users, particularly those who did not attend a clinic and could not be selected as cases. These two groups may differ in their characteristics. In addition, the study focused on Lahu youths; therefore, it is unclear whether the findings can be applied to other tribes who are living in the same areas. One more significant point of the study is the illegal status of MA in Thailand, leading some people to not disclose their MA use. This could be one of the concerns of misclassification in the present study, particularly in the control group. Finally, although all selected cases were willing to participate in the study, three selected cases had moved to other provinces and could not be reached for further information.

Conclusion

Several individual, interpersonal, and structural factors contribute to MA use among Lahu youths. Females, no schooling, unemployed status, high knowledge of MA, and substance use behaviors are associated with MA use among Lahu youths. Creating a national policy for promoting access to education among Lahu youth who are living in remote areas and special training programs that fully engage local people to improve their preventive behaviors in MA prevention and control should be carefully created and implemented in Lahu villages, which are located in remote and border areas. Moreover, existing laws and regulations related to reducing the availability of MA and substances in the villages could be complemented to reduce the size of the problem. For further research, a community-based quasi- or full experimental study focusing on preventing new MA use among Lahu and other hill tribe youths should be conducted.

Supporting information

S1 Appendix. Question guide.

(DOCX)

pone.0344775.s001.docx (236.5KB, docx)
S1 File. STROBE.

(DOC)

pone.0344775.s002.doc (81.5KB, doc)

Acknowledgments

We extend our appreciation to the Provincial Public Health Office, District Public Health Office located in Mae Suai, Mae Chan, Mae Fah Luang, and Muang District, village health volunteers, village heads, research participants, and their parents for their assistance and cooperation in data collection.

Data Availability

Data cannot be shared publicly because it contains highly sensitive information. Data are available from the Center of Excellence for Hill Tribe Health Research, Mae Fah Luang Univeristy Institutional Data Access Committee https://cehr.mfu.ac.th/publications.html (cehr@mfu.ac.th) for researchers who meet the criteria for access to confidential data.

Funding Statement

The author(s) received no specific funding for this work.

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Decision Letter 0

Rining Wei

27 Jun 2025

Dear Dr. Apidechkul,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

Reviewer #1: Partly

Reviewer #2: Yes

Reviewer #3: Partly

**********

2. Has the statistical analysis been performed appropriately and rigorously? -->?>

Reviewer #1: Yes

Reviewer #2: No

Reviewer #3: No

**********

3. Have the authors made all data underlying the findings in their manuscript fully available??>

The PLOS Data policy

Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English??>

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

Reviewer #1: Dear authors,

Thank you for your important work addressing methamphetamine (MA) use among Lahu youth—a critical public health issue with significant societal implications. Your manuscript is a sound technical piece. However, you can still improve it more to convince the reader that your methods were rigor. Few more information about the methods and materials will do that for you :-). Several statistical findings on questionnaire validation data, pilot tests, sensitivity tests and VIF will enhance the availability of your data. Overall, your paper is well structured. But, further synthesis (e.g., merging repetitive points) and explicit limitations would bolster its impact. Therefore, please find the detailed review sheets for your further perusal and wishing you all the very best! I appreciate your dedication to this vital topic and thank you for the opportunity given to read and review your paper.

Reviewer #2: PONE-D-25-27632

Factors influencing methamphetamine use among Lahu hill tribe youth Chiang Rai, Thailand: A case‒control study

• In the following page and line numbers will be given as: page.#

The printed page in the file will be used for the body of the text for page number.

• Edit suggestions (some stylistic, some substantive) will be given as: a => b (change a to b). [my explanations as to why appear in square brackets]

Review

This research investigates factors influencing methamphetamine use among Lahu hill tribe youth in norther Thailand using a case-control study with 136 cases (using) and 272 controls (not using), mostly male (79.4%) across four districts of Chiang Rai Province. It is an important topic.

Various predictors are tested for significance between cases and controls and a regression model employed thereafter. It is not clear why the variables were specifically chosen in the logistic regression given there are many that are significant in the univariate comparison between cases and controls. (One can’t include everything, but articulate why particular ones are chosen. Perhaps one his highly correlated with others, so it stands as a proxy for several – for example the similar significance of use of Twitter (X) and Instagram by controls (about 1 in 5) but much less by cases (p <0.001).

Several predictors were identified as significant in the model, but the confidence intervals are exceedingly wide around the estimate (for example an odds ratio of 6.40 for No schooling had a 95% CI of (1.12, 34.17) and Heroin Use had an odds ratio of 26.85 and 95% CI of (4.17, 172.95).

From a distance, the wide CI’s could be due to two or more predictors in the model having high correlation resulting in multicollinearity which will inflate the confidence intervals. Where high correlation between two predictors is the case, one predictor may more or less stand as a proxy also for the other giving justification for eliminating one from the model.

Given this possibility the authors should run a correlation matrix for the possible predictors in the model and check whether a correlation of 0.7 or more exists between two or more predictors. If logically related one can drop one. This should tighten the confidence intervals. One could run it with one and then the other to see how each impacts the other variable results as well – without too much fishing. Also one variable might be more important for the context thematically which might inform which to keep even if it is slightly less predictive. It may be that two logically unrelated predictors have high correlation by chance, which leaves one a choice on whether or not to eliminate one. If both being in inflates the Cis, then one should likely eliminate one of the predictors, noting their correlation in any case.

Whatever model is settled on, do mention the predictor that was dropped and its correlation level with the one kept if a reader might ask why a seemingly important variable found significant in a simple comparison between cases and controls was not in the multivariable model.

• Insert a table with AOR, 95%CI, p-values only for the multivariable logistic regression results where all of the included variables and results are together rather than results in the long list of univariate results separated far from each other in the long Table 4 (Factors, pages 16-24). That will make it much easier to read, edit, and comment on the logistic regression predictor selections and results.

• Also move Variables [Lines 5.25-28] on variables in the multivariate logistic regression to just before the new table after Table 4 with only with regression results. Note that there are many independent variables (possible predictors) that are significant but only a subset are included in the model. State how were they selected for the model (for example no social media variable was included although some are very significant univariately). See note above also concerning collinearity.

• When “controlling for age” in the multivariable model, is age the actual age of the person in years or the categorical Age variable listed as 15-19 and 20-24 years? (If actual age was there a coefficient and significance for the change in the log-odds of the outcome for each year above age 15; if categorical, was there an AOR for the variable?)

• Concerning Data Availability “No – some restrictions will apply” – explain the restrictions (what and why).

• On the Table called Factors covering pages 17-24 the percentages in parentheses should sum to be 100% vertically, not horizontally. That most are about 1/3 to 2/3 horizontally is due to the fact that there are only half as many cases as controls. Given different denominators 33.3 and 66.7 horizontally actually means 50% of cases and 50% of controls, which would show as 50.0 and 50.0 if summed vertically. The percentages for all Cases and Controls in the table have to be summed vertically.

• Page 27: A section labeled “Limitations” needs to be added after the Discussion. How generalizable are the results among all Lahu youth? For example the cases all have been in a treatment program but some MA users are not in a treatment program. Could there be differences between the MA users in treatment and those not reaching a treatment program? (How does one get into a treatment program?) Would one expect similar results in other hill tribe groups? Do the results compare with studies of other hill tribe groups (if there are such studies)? What are some limitations and advantages of Case-Control design in general?

Abstract

Background:

• 1.23 [page1.line23]: social insecurity => social security

[ impact on “health” before is a positive term, so impact on security keeps the second consistent]

• 1.24-25: [are the Lahu the second largest hill tribe in Thailand/northern Thailand? Or are Lahu youth identified as the second largest marginalized subpopulation within the Lahu tribe? If so which is the largest hill tribe marginalized population? Clarify.]

• [Perhaps:] 1.25: population => subpopulation

Results:

• [see comment above about large confidence intervals as indicating possible multicollinearity from correlated predictors in the logistic regression model – rerun model after if finding this to be the case and eliminating a predictor]

• 2.2: “After controlling for age”

[is actual age included as a continuous variable, is the two-level categorical variable Age (15-19, 20-24) used? is there a coefficient and significance related to Age in the regression model? The univariate comparison for age as a categorical variable was not significant.]

Conclusions: [Good]

Keywords: [Good]

Introduction:

3.9-10: part of the route of MA import, which is imported from neighboring countries,

=> part of the route of MA import from neighboring countries, then

3.11: and being => or being

3.13: uneconomic => subsistence

3.16: live under => live below [just sounds a little better]

3.18: including health care services

=> including health care services, because [tell why not granted a Thai ID card]

3.21: to MA use, => for MA use,

3.22-23: This study aimed to => The present study aims to

Methods:

3.32: case-control => case-control study

4.1: the odds ratio from the previous study => the ratio from a previous study

[ratio of controls to cases not odds-ratio? … it sounds like a simple ratio.]

4.2: a total of 136 cases and 272 controls

=> a total of 136 cases and 272 controls (a control/case ratio of 2.0)

4.8: been recorded in the MA treatment program as a client.

[Could Lahu MA-using youth recorded in a MA treatment program differ from Lahu MA-using youth who never make it to a treatment program? Mention this possibility or address this in the Discussion or under Limitations.]

4.25: gender with three options for their response: male, female, or other

[no respondent indicated ‘other’ for Gender in Table 1 on page 7. In the text ‘sex’ is used as well as gender. I would use only use ‘gender’ to be consistent with the questionnaire unless meaning biological sex exactly and not social/psychological identification. Also in Thai language is there a difference between “sex” and “gender” in the word used in the questionnaire? Among Lahu people generally (in contrast to academic researchers) “gender” may simply be understood as biological sex.

4.26: social media sue => social media use

5.25-28 [Section on Variables … comment above on moving these lines to just before a table with multivariate logistic regression results collected together in one table only with selected predictors.]

5.29-6.17 Recruitment procedure [good section on Recruitment procedure]

6.18-27 Statistical analysis [Good section – other comments on variable selection elsewhere.]

Note that Pseudo R-squared of logistic regression does not indicate proportion of variation of the dependent variable explained by the independent predictors as is the case of R-squared for linear regression. Pseudo R-squared in logistic regression context indicates the relative improvement in model fit compared to a baseline model (often a null model without predictors but only an intercept – that is the overall proportion in the data of the dependent variable being = 1 (which is trivial in a Case-Control set up where the cases = 1 and comprise one-third of the data by design as 2 controls = 0 have been included for every case such that controls =0 are two-thirds of the data).

For the Hosmer-Lemeshow test a high p-value is good (better fit) and a low p-value indicates a bad fit. This test, which uses a chi-squared approach, has been shown to be sensitive to the number of data partitions it uses (generally 10 in most software packages by default). A different number of partitions gives a different significance level for the same data. For this reason some researchers advise against using it although it is used in SPSS. It does provide some indication of goodness of fit of the model.

• 6.28-7.2: Ethical approval and consent to participate [Good section]

• 7.3: Results

• 7.13-9.1: Table 1 Comparisons of general characteristics between case and control [Table is OK]

• 9.7: [New paragraph - indent]

• 9.7: Regarding Substance use behaviors => Regarding Substance use behaviors in Table 2 below,

• 9.15-16: Facebook (78.2%) and Line (69.9%) were the most commonly used social media platforms.

=> Facebook (78.2%) and Line (69.9%) were the most commonly used social media platforms, but use levels were not significantly different between cases and controls.

• 9.16-17: sharing personal challenges with family members (p value<0.001)

[move to bottom of paragraph]

• 9.16-17: sharing personal challenges with family members (p value<0.001)

=> sharing personal challenges with family members was stark in contrast with only 36.0% of cases doing so compared to 62.5% of controls (p value<0.001) – cases were much more isolated from family in communication.

• 9.17-18: using Instagram (p value<0.001), and using Twitter (p-value<0.001) (Table 2).

=> While about 1 in 5 controls used Twitter (X) and Instagram, cases used both much less (2.9% and 5.1%) both with p-value<0.001 (Table 2).

• For long tables, see if there is a box for repeating the table column labels at the top of each page if the table covers multiple pages.

• 12.2: Two-thirds (71.6%) of the participants had their parents living together.

[Move this to Page 7 for discussion of Table 1 which has this variable.]

Two-thirds (71.6%) of the participants had their parents living together.

=> The parents of cases lived together 18% less than controls (p<.001).

• 12.2: Half of => In Table 3 below, half of

• 12.16 (Table 3). => Cases had much higher relative access to cannabis, opium, and heroin than Controls (all p<.001).

• Page 13: [note “Having been persuaded by friend to use MA” has 9.6% of Controls saying “Yes” which should be 0% by the inclusion criteria. Perhaps the 9.6% of cases indicates that a friend attempted to persuade them but did not succeed.]

• 15.11: [How were the predictors selected for inclusion in the multiple logistic regression model given so many significant univariate variables were not included?]

• 15.11: [Is age the actual age in years or the two-level categorical variable from Table 1? Did age have a coefficient or odds ratio and a significance level?]

• 15.11-15.22: [See comment above about inflated confidence intervals (CI’s) and the need to check for collinearity and rerunning model with collinearity reduced.]

• Page 16: [Table 4 should start on this page beneath the heading. Almost blank page.]

• [For Table 4 AOR, 95%CI, p-value for the multivariate logistic regression can be gathered into a separate table for the model which is much more easy to read.]

• Pages 17-23: [The variable %’s in parentheses are now summed horizontally for each Factor in Table 4. These must be summed vertically to sum to 100%. Horizontally the cases are mostly about 1/3 and controls about 2/3 as that is the proportion of the design.]

• Page 24: The significance note should appear on Page 23 under the end of the table.

Page 25: Discussion

• 25.2: use, including => use including [delete comma]

• 25.3: high knowledge of MA and control,

=> high knowledge of MA prevention and control,

• 25.3: and being => being

• 25.13: due to their health literacy => due to their health literacy being low

• 25.15: compulsory => necessary

• 25.16: Those => Seemingly paradoxically, those

• 25.24: prevention and control.

=> prevention and control. MA users not attending such clinics may also have a much lower level of knowledge as well.

• 25.27-28: smoking hill tribes, people living => hill tribes, people smoking living

• 25.28: did not. => did not smoke.

• 26.16: school. We also found that having no family member to share a personal challenge was true for 2/3 of the cases compared to 1/3 of the controls.

• Lines 26.23-31 [move to section titled “Limitations” perhaps after 27.8 or before Conclusion at 26.32]

• 27.1: organizational layers => structural layers

• [Insert blank line after 27.8]

Page 27: A section labeled “Limitations” needs to be added after the Discussion. Move Lines 26.23-31 under Limitations.

• 27.12: my => our

• 27.14: village head => village heads

• 27:14: participant => participants

• 27.18: P Phootawan => Phootawan

References [good]

• 30.2: thai => Thai

• 30.10: Lahu ethnical, Thailand => [Lahu ethnicity, Thailand]

[square brackets for title as original is in Thai]

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Thank you for the opportunity to review this important research.

Reviewer #3: I want to commend the authors for their work on this relevant topic. The research tackles a critical public health issue affecting underprivileged populations and demonstrates interesting innovations, such as the creative triple-check control verification method and rigorous objective biomarker validation—both of which advance the epidemiological science among marginalized communities. Please find below my suggestions and concerns, organized point-by-point.

Introduction:

• I'm curious about the national identification card system in Thailand, as this emerges as such an important factor in your findings. It would be helpful to understand the circumstances under which Lahu community members might not receive national identification cards and the implications this has for accessing services.

• Your introduction does an excellent job establishing the MA use challenges among Lahu youth. I wonder if you might consider sharing additional insights about other socio-cultural characteristics of this population? This could help readers like myself better understand the broader environment that contributes to these patterns.

Methods:

• The sample size calculation references a "previous study" (Thailand narcotics control annual report 2023). While the calculation is provided, it would enhance clarity to briefly explain how this reference study relates to the current methodological approach and justify the assumptions used.

• For improved readability, consider reorganizing the methods section by placing “recruitment procedure" immediately after "study sample," potentially subdividing this into "inclusion criteria" and "recruitment procedures." This would maintain the logical flow of understanding sample construction.

• If possible, I believe a participant flow diagram would benefit overall comprehension for the case-control by showing the selection process clearly.

• Regarding case selection from MA clinics: could the authors clarify whether this refers to a single clinic or multiple facilities? Additional context about the clinic setting(s) would help readers understand the representativeness of the case population.

• Your control selection process sounds very thorough. I am still unsure whether controls were drawn from the entire eligible Lahu community aged 15-24, or if there were additional selection considerations. You might also consider moving that first results paragraph (with the final sample sizes) into methods—it would really strengthen the transparency of your approach.

• While sex-matching is well-justified given known gender differences in MA use, could you elaborate on the decision not to match on other factors like age range? The rationale for matching choices is crucial in case-control design and would contribute to the methods description.

• The authors write that “There were several independent variables or predictors in the study, e.g., age, sex, marital status, education, occupation, family income, knowledge of MA prevention and control, and attitudes toward MA prevention and control.” I would suggest that they either clearly define the variables rather than putting out examples of what may appear in the study. For instance, STROBE reporting guidelines (doi:10.1371/journal.pmed.0040296) suggest we “clearly define all outcomes, exposures, predictors, potential confounders, and effect modifiers”, and the authors do provide some comprehensive description for variables in the “research instruments” sections. However, it’s preferable to have an objective perspective on variables used in the study, with clear definitions if not previously done. I suggest the authors revise their instruments and variables section to avoid redundant content and explicitly – and intentionally – present the selected variables, their treatment and application (e.g. outcome, covariate, exposure).

• I'm interested in your 15,000-baht household income threshold. Could you help us understand how this relates to Thai socioeconomic standards? I noticed you mention Thailand's national poverty line of $1,400/year per family in the introduction—connecting these would perhaps help.

• I think there might be a small calculation issue in Table 1's household income row (the percentages appear to sum to 109%)—just wanted to flag this for your review.

• Please clarify whether missing data analysis or sensitivity analyses were conducted. If not performed, this should be explicitly stated. If conducted, the patterns and handling methods would be valuable to report.

Results:

• Consider streamlining the presentation by reducing repetitive "p value" references throughout the text.

• The final (adjusted) multivariate model controls only for age despite multiple significant univariate associations (education, occupation, income, family status). Could you discuss the rationale for this approach and potential implications for residual confounding?

• Could you clarify whether you included age as a continuous variable or in categorical groups in your multivariate analysis? Just a small detail that would be helpful.

• Given the cross-sectional measurement of exposures and outcomes, could you address the temporal sequence challenges, particularly for variables like "knowledge of MA prevention and control"? This pattern suggests the knowledge was acquired through treatment programs after MA use initiation, rather than representing a causal pathway to use. For clarity, I would recommend the authors explicitly state that their counterintuitive findings regarding increased use associated with MA knowledge may be caused by reverse causation bias: knowledge came after MA use, not before.

• Some effect sizes appear unusually large with very wide confidence intervals (e.g., heroin use: AOR 30.14, CI: 4.29-211.95; friend persuasion: AOR 13.79, CI: 3.80-50.05). These suggest either very small cell sizes or potential data quality issues. Could you provide the actual numbers in these cells and discuss the reliability of these estimates?

Before concluding, I would also recommend the authors consider discussing study limitations more comprehensively, including potential selection biases, temporal sequence issues, and generalizability beyond this specific population.

**********

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Reviewer #1: No

Reviewer #2: Yes: Kirk C Allison

Reviewer #3: Yes: Cézar D. Luquine Jr.

**********

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pone.0344775.s003.pdf (3.9MB, pdf)
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PLoS One. 2026 Mar 12;21(3):e0344775. doi: 10.1371/journal.pone.0344775.r002

Author response to Decision Letter 1


21 Aug 2025

Response to editor and reviewers comments

PONE-D-25-27632

Factors influencing methamphetamine use among Lahu hill tribe youth Chiang Rai, Thailand: A case‒control study

PLOS ONE

Dear Dr. Apidechkul,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

==============================

Please take into account each of the referee suggestions and revise accordingly.

Looking forward to your revised version!

==============================

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Additional Editor Comments :

Please take into account each of the referee suggestions and revise accordingly.

Comments to the Author

Reviewer #1: Dear authors,

Thank you for your important work addressing methamphetamine (MA) use among Lahu youth—a critical public health issue with significant societal implications. Your manuscript is a sound technical piece. However, you can still improve it more to convince the reader that your methods were rigor. Few more information about the methods and materials will do that for you :-). Several statistical findings on questionnaire validation data, pilot tests, sensitivity tests and VIF will enhance the availability of your data. Overall, your paper is well structured. But, further synthesis (e.g., merging repetitive points) and explicit limitations would bolster its impact. Therefore, please find the detailed review sheets for your further perusal and wishing you all the very best! I appreciate your dedication to this vital topic and thank you for the opportunity given to read and review your paper.

Review Report: Factors influencing methamphetamine use among Lahu hill tribe youth Chiang Rai, Thailand: A case‒control study

Dear authors,

It is remarkable that you have clearly pointed out MA as a public health issue, and the Lahu youth population is vulnerable to that. Yet, I would like to clear some concerns and provide several comments to further enhance the understandability of the manuscript.

Abstract:

The abstract is well structured. However, there are some minor concerns from my side, which I might need a little clarification or a slight modification to the written abstract. Please address them accordingly:

1.Under the methods: “... The participants were recruited from...”. Could you please clarify the sampling method here?

: We used a random method to recruit the sample. Please see page 1, line 31.

2.The authors stated, “A validated questionnaire was used to collect data, and multiple logistic regression was used to detect association factors at α = 0.05.” The use of the term “detect association factors at α = 0.05” is understandable. However, to make it technically sound, would you consider rewording the statement to provide an idea like “...with statistical significance set at α = 0.05...”? Such a clear expression would avoid any ambiguity.

: Thank you for the comment. We agree with you and have revised accordingly. Please see page 1, line 34.

3. When I was referring to the data collection instrument of your study, I noticed that the expected outcomes of your study are not always binary. If the outcome measures are binary, it is technically accurate to use the “multiple logistic regression”; otherwise, it is recommended to use linear or ordinal regression for the data analysis. Please recheck your data analysis plan and supplement with related information in your methods section.

: Thank you for the comment. Our study outcome was whether to use AM or not, which was set at the beginning. Then we used the logistic regression instead of linear regression.

Introduction:

Informative approach in introducing the scope of your research area and the aims. You have mentioned the neurobiological effects of using MA and also the societal consequences. Special emphasis on the socio-economic disparities in the context of Thailand's geographical risks is an important fact.

4. However, the lines 11 to 19 do not create a bridge between the MA use with the socio- economic disparities that you have emphasised there, including the ‘education’. Could you please elaborate on the association between the MA use and the mentioned socio- economic factors in the lines? Or could you briefly mention why Lahu youth are at higher risk by referring to the socioeconomic factors and geographic isolation?

Thank you for the great comment. We have extended details in page 3, lines 18-20.

5. It is appreciated that you have cited sources from both the Thai-originated sources and the international sources. When considering the logical flow of your introduction, it can be guessed that you have focused first on bringing the readers’ attention to the global scenario, then Thailand, then Chiang Rai in Thailand, and the Lahu youth. However, it would be better if you could also add a paragraph to emphasise the gaps in the literature, which should be a critical element of an introduction to a manuscript. Because, in your introduction, it was not explicitly stated why previous research was not sufficient, or why the previous research was not available in your area of interest as a controlled case study.

: Thank you for your excellent comment. We have added information in page 3, lines 24-28.

6.Lines 22 to 24: You mentioned the study objective. However, there was no justification in the introduction on why the case-control method was selected. Could you briefly mention why you have selected the case-control study design? It is highly recommended to clarify/state the hypotheses rather than hinting it in a descriptive way through the research aim. Please consider modifying the way you report the aims by giving more focus into your study’s hypotheses.

: Thank you for the comment. We totally agree with your comment that to guide the direction to readers, we need a hypothesis. Please see page 4, lines 3-5.

7.It is suggested to use your statements in lines 20-22 in the discussion section rather than in the introduction section.

: Thank you for the comment. With the context of the existing sentence focused on general discussion, we have modified the paragraph to be the basis of the idea to raise the study objective in this paragraph. Thank you very much. Please see lines page 3, 24-29,

8.Because your overall conclusion is to emphasise the need for “effective interventions and changes in policy” the introduction must expand a bit its concluding remarks by supplementing information about the policy relevance of this study.

: Thank you for such a great idea. We have extended the information on page 3, lines 29-30.

Materials and Methods:

The methods section is satisfactorily planned. However, the following comments/ questions will help you to increase the readability and broaden the understanding of your study for readers with less understanding of statistical methods. Please consider modifying your writing based on the following:

9.Line 27: The same comment (comment 3 of the introduction) applies here. You may address it under study design.

: Thank you. Please see the information added on page 3, lines 28-30.

10.Lines 29 to 30: Please consider using full sentences. Could you rewrite for better readability?

: Thank you for the comment. It is revised and completed. Please see page 4, lines 3-5

11.The sample size was calculated appropriately. However, to ensure expanded readability, it is better if you could add notes for the mentioned variables/constants/functions/symbols in the equation. This would help readers who may not have a wider understanding of equation-related statistical symbolic expressions.

: Thank you, we have added information in all notes used.

12. Line 10 onwards: To enhance the objective verification, could you mention whether there were complementary self-reports compared to the urine test results? If the results were not complementary, what was the next step?

: Thank you for the great comment. We have revised the sentences in lines 17-21 on page 4 to ensure that the information has been cleared for readers. Individuals who responded to any of the three methods with “Yes” were assumed to be a case (MA user).

13.The triple triple-check method is obviously a strong method to ensure the selection validity of your sample. Yet, despite the triple-check, there might still be social desirability bias among the controls as the MA use can be under-reported because of its stigma. Could you please try considering this as a fact when addressing the previous comment, too?

Thank you. We do know that with the conditions of illegal MA use in Thailand and also social desirability bias, which could be either a higher or lower possibility to say “yes” or “no” for some people, but it is the best way that we could do at the moment. We have put our limitations of the study on page 24, lines 17-32. Please also see my response to question No.14.

14.Also, the controls from the same village might share unmeasured confounders such as access to the MA. How have you addressed this as a fact?

: We selected cases and controls randomly after being classified by the triple method, and before the interview, they did not know about the process of identifying/assessing, including the questions asked, which are common to a case-control study. We

Attachment

Submitted filename: Respone to reviewer comments .pdf

pone.0344775.s006.pdf (204.6KB, pdf)

Decision Letter 1

Steve Zimmerman

28 Oct 2025

Dear Dr. Apidechkul,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

--> -->-->However, each of the three reviewers still has some requests for further improvements to the manuscript - please see their comments below.-->--> -->-->I also have a concern with the categorisation of continuous variables, specifically the knowledge and attitude scores, although this applies to age as well.-->--> -->-->You have dichotomised or trichotomised these variables for the logistic regression analysis. However, you have not provided a reason why knowledge and attitude scores should be split into three categories, or why the cutoffs should differ across these two measures. None of the sources you cite (refs 29-31) treat continuous variables in this manner (with the exception of a single analysis in ref 29). Unless there is good reason to do so (e.g., bimodal data), continuous variables should not be changed into categorical variables. Knowledge, attitude, and age can all be entered as continuous predictor variables in logistic regression. Please could you re-run your analyses with continuous predictors where relevant/appropriate?-->--> -->-->Could you also please carefully revise the manuscript to address all comments raised by the reviewers?-->--> -->-->?>

Please submit your revised manuscript by Dec 12 2025 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org . When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

  • A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

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If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

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We look forward to receiving your revised manuscript.

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Steve Zimmerman, PhD

Senior Editor, PLOS One

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[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

Reviewer #3: (No Response)

**********

2. Is the manuscript technically sound, and do the data support the conclusions??>

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Partly

**********

3. Has the statistical analysis been performed appropriately and rigorously? -->?>

Reviewer #1: (No Response)

Reviewer #2: Yes

Reviewer #3: I Don't Know

**********

4. Have the authors made all data underlying the findings in their manuscript fully available??>

The PLOS Data policy

Reviewer #1: (No Response)

Reviewer #2: Yes

Reviewer #3: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English??>

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

Reviewer #1: Dear Authors,

Thank you for responding to the review comments and being patient enough with providing further clarifications. There is one more suggestion from my side. Could you please consider replacing the statement "Missing data were checked before further analysis" to indicate that missing data were not reported?

Wishing you all the very best!

Reviewer #2: PONE-D-25-27632 Resubmission Review

Thank you to the authors for the responsive revisions.

The major issues have been addressed in the revision including correction of tables column summing and introduction of a separate table for the multivariate analysis (Table 5). The issue of the wide confidence intervals for certain variables was investigated concerning possible multicollinearity (with one variable dropped) using variable correlation and VIF (variance inflation factor) investigation. Small cell size was also identified as a possible culprit.

Below are a list of minor edits from reading the revised manuscript. I do not need to see these edits. Here again page and line # is indicated as page.line#. a=>b means change a to b. [Comments in brackets simply clarify the change.]

- Throughout : p value or p-value

[as the editor prefers … in the US it is mostly p-value]

- 4.28: :: => : [delete one colon]

- 7.19: drop => dropped

- 7.24: were => was [singular, agrees with ‘reliability’ as subject]

- 8.9: determine => examine

- 21.9: fold( => fold ( [insert space]

- Page 22 Table 5: [add * after Yes/Ever p-value concerning Heroin use]

0.001 => 0.001*

- 23.16: Either smoking or heroin use, was identified

=> Both smoking and heroin use were identified

- 23.22: particularly young adults => particularly among young adults

- 23.26: use of cod heroin and MA => use of codrug heroin and MA

or: => use of heroin and MA [not sure what ‘cod’ means]

- 24.11: MA access => MA access near borders

- 24.24: difficult to => unclear whether

- 25.9: laws => existing laws

[as in complement existing laws? Or perhaps meaning strengthen existing laws?]

Thanks again for working on this important area.

Reviewer #3: I thank the authors for their thorough effort in responding to our feedback. It shows that you have tried to answer to the variety of suggestions and comments reviewer put forward. While you've demonstrated understanding of the methodological concerns raised, a significant disconnect exists between your explanations to reviewers and what appears in the manuscript. Readers would greatly benefit from the clarifications you provided in your responses being incorporated into the manuscript text.

Your responses contained valuable explanations for: sample size calculation rationale based on the Thailand narcotics control report; the logic behind different income thresholds (15,000 baht for graduates vs. $1,400 poverty line); your data completion procedures that minimized missing values; the rationale for age-only adjustment in the final model; and the categorization of age variables. These explanations must be incorporated into the manuscript as they address fundamental questions readers may have about your methodology.

Additionally, while you explained age adjustment to reviewers, the final model's exclusion of other significant univariate factors (education, occupation, income, family status) despite your acknowledgment of “wide CIs due to small cells” suggests model instability rather than appropriate selection. Consider reporting exact cell counts for problematic variables and discussing precision limitations explicitly or use formal model selection criteria to justify your approach.

**********

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Reviewer #1: Yes: D.L.I.H.K. PEIRIS

Reviewer #2: Yes: Kirk C Allison

Reviewer #3: Yes: Cézar Luquine

**********

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PLoS One. 2026 Mar 12;21(3):e0344775. doi: 10.1371/journal.pone.0344775.r004

Author response to Decision Letter 2


12 Nov 2025

Response to reviewers comments

Dear Editor and reviewers,

Thank you very much for the opportunity to revise again with valuable comments and feedback from you. We have carefully revised based on all your comments and feedback. We re-analyzed the whole data set by treating age and knowledge in continuous forms, and the final model looks better than the previous one. Please see details on our feedback below, and very hope you are happy with this version.

Thank you,

TK

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

The three original reviewers have assessed your revised manuscript and thank you for addressing the issues they raised during the first round of review.

However, each of the three reviewers still has some requests for further improvements to the manuscript - please see their comments below.

I also have a concern with the categorisation of continuous variables, specifically the knowledge and attitude scores, although this applies to age as well.

1. You have dichotomised or trichotomised these variables for the logistic regression analysis. However, you have not provided a reason why knowledge and attitude scores should be split into three categories, or why the cutoffs should differ across these two measures. None of the sources you cite (refs 29-31) treat continuous variables in this manner (with the exception of a single analysis in ref 29). Unless there is good reason to do so (e.g., bimodal data), continuous variables should not be changed into categorical variables. Knowledge, attitude, and age can all be entered as continuous predictor variables in logistic regression. Please could you re-run your analyses with continuous predictors where relevant/appropriate?

: Thank you very much for the comments. We have re-analyzed the whole process in a stepwise with pr.(0.2) for selection variables into the model, and treated age and knowledge in continuous forms. The final model, ten (10) variables were retained in the model for predicting, and seven 97) variables were found to be associated with MA use. We have revised all relevant information, including Table 5. Please see page 2, lines 3-13; page 22, lines 3-15, table 5, discussion and conclusion sessions.

We do hope that you are happy with this version.

Reviewers' comments:

Reviewer #1: Dear Authors,

2. Thank you for responding to the review comments and being patient enough with providing further clarifications. There is one more suggestion from my side. Could you please consider replacing the statement "Missing data were checked before further analysis" to indicate that missing data were not reported?

: Thank you for the comment. We have modified the statement to “Missing data were checked and managed properly before further analysis.” Please see page 7, lines 29-30.

Reviewer #2: PONE-D-25-27632 Resubmission Review

Thank you to the authors for the responsive revisions.

The major issues have been addressed in the revision including correction of tables column summing and introduction of a separate table for the multivariate analysis (Table 5). The issue of the wide confidence intervals for certain variables was investigated concerning possible multicollinearity (with one variable dropped) using variable correlation and VIF (variance inflation factor) investigation. Small cell size was also identified as a possible culprit.

Below are a list of minor edits from reading the revised manuscript. I do not need to see these edits. Here again page and line # is indicated as page.line#. a=>b means change a to b. [Comments in brackets simply clarify the change.]

3.- Throughout : p value or p-value

[as the editor prefers … in the US it is mostly p-value]

: Thank you for the notice. We totally agreed with you and have changed to p-value all points used in the paper.

4.- 4.28: :: => : [delete one colon]

: Thank you, it’s improved

5.- 7.19: drop => dropped

: Thank you, it’s improved

6.- 7.24: were => was [singular, agrees with ‘reliability’ as subject]

: Thank you, it’s improved

7.- 8.9: determine => examine

: Thank you, it’s improved

8. - 21.9: fold( => fold ( [insert space]

: Thank you, it’s improved

9.- Page 22 Table 5: [add * after Yes/Ever p-value concerning Heroin use]

0.001 => 0.001*

: Thank you, it’s improved

10. - 23.16: Either smoking or heroin use, was identified

=> Both smoking and heroin use were identified

: Thank you, it’s improved

11. - 23.22: particularly young adults => particularly among young adults

: Thank you, it’s improved

12. - 23.26: use of cod heroin and MA => use of codrug heroin and MA

or: => use of heroin and MA [not sure what ‘cod’ means]

: Thank you, it’s improved, it is “codrug..”

13. - 24.11: MA access => MA access near borders

: Thank you, it’s improved

14. - 24.24: difficult to => unclear whether

: Thank you, it’s improved

15.- 25.9: laws => existing laws

[as in complement existing laws? Or perhaps meaning strengthen existing laws?]

: Thank you, it’s improved

Reviewer #3: I thank the authors for their thorough effort in responding to our feedback. It shows that you have tried to answer to the variety of suggestions and comments reviewer put forward. While you've demonstrated understanding of the methodological concerns raised, a significant disconnect exists between your explanations to reviewers and what appears in the manuscript. Readers would greatly benefit from the clarifications you provided in your responses being incorporated into the manuscript text.

16. Your responses contained valuable explanations for: sample size calculation rationale based on the Thailand narcotics control report; the logic behind different income thresholds (15,000 baht for graduates vs. $1,400 poverty line); your data completion procedures that minimized missing values; the rationale for age-only adjustment in the final model; and the categorization of age variables. These explanations must be incorporated into the manuscript as they address fundamental questions readers may have about your methodology.

: Thank you for such great comments. We have added information on the main manuscript. Please see page 4, lines 10-11/ page 7, lines 33-34/ page 8, lines 1.

17. Additionally, while you explained age adjustment to reviewers, the final model's exclusion of other significant univariate factors (education, occupation, income, family status) despite your acknowledgment of “wide CIs due to small cells” suggests model instability rather than appropriate selection. Consider reporting exact cell counts for problematic variables and discussing precision limitations explicitly or use formal model selection criteria to justify your approach.

: Thanks for the great comment. We have revised the whole process of the analysis again using the stepwise method at selection propagability pr.(02) after putting all variables in the model and treating knowledge and age at continuing forms as recommended. It was found that 10 variables were found to be associated with MA use. Among the remained variables in the stepwise mode, three variables presented in the previous version were not added as the predictors: heroin use, having been persuaded by a friend, and smoking cigarettes.

While another six variables were added into the model as the predictors for the MA use: sex, cigarette accessibility, marital status, parent living status, occupation, and alcohol assumption (see Table 5). We have seriously considered together among the authors and agreed that this new model is much better than the previous one and is be used as our final model. At the end, seven (7) variables were associated with MA use. Then, we have modified the relevant context and added information in the discussion part. We thank you so much for these great suggestions. Please see page details in Table 5, and page 22, lines 3-15.

Thank you,

TK

Tawatchai Apidehckul

Attachment

Submitted filename: MA_Reviewers commenst.pdf

pone.0344775.s007.pdf (46.3KB, pdf)

Decision Letter 2

Aditya Pawar

11 Dec 2025

Dear Dr. Apidechkul,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Please submit your revised manuscript by Jan 25 2026 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org . When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

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Aditya Pawar

Guest Editor

PLOS One

Journal Requirements:

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Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

Additional Editor Comments:

Kindly address the final comments by Reviewer 2.

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions??>

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously? -->?>

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available??>

The PLOS Data policy

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English??>

Reviewer #1: Yes

Reviewer #2: Yes

**********

Reviewer #1: Dear Authors,

Thank you for going through revisions and making the paper clearer. I do not have further comments.

Reviewer #2: Thanks for taking care of the minor edits.

With apology concerning the regression model in Table 5 - one more run please:

The first revision regression model included is Cigarette Smoking with a confidence interval of 2.34-25.62 (p-value <0.001*)

in the second revision the regression model included two overlapping variables concerning smoking.

Cigarette Smoking: with a confidence interval of 11.57-73.612 (p-value <0.001*)

Cigarette Accessibility: with a confidence interval of 0.25-229.55 (p-value 0.241)

While Cigarette Smoking is significant in both the first and second revision, the second revision Cigarette Smoking has a much broader CI than the first revision while the overlapping variable Cigarette Accessibility has a massively large CI range 0.25-229.55. Likely co-linearity again.

Please rerun the regression model for Table 5 with either Cigarette Smoking or Cigarette Accessibility alone but not both. One could try each one separately for interest. My hunch is either will be significant and the CI range much narrower.

With appreciation for your work.

**********

what does this mean? ). If published, this will include your full peer review and any attached files.

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Reviewer #1: Yes: D.L.I.H.K. PEIRIS

Reviewer #2: No

**********

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PLoS One. 2026 Mar 12;21(3):e0344775. doi: 10.1371/journal.pone.0344775.r006

Author response to Decision Letter 3


15 Dec 2025

Response to reviewers comments

Dear Editor and reviewers,

Thank you very much for the opportunity to revise again with very clear point for improvement. We have re-analysis and revised the paper as suggested. All references have been double checked to ensure that are align with the journal requirement. Please see the new submitted paper and very hope you happy this version.

Thank you,

TK

Editor

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

:Thank you very much, we have carefully checked all references before this submission

Reviewer #2: Thanks for taking care of the minor edits.

With apology concerning the regression model in Table 5 - one more run please:

The first revision regression model included is Cigarette Smoking with a confidence interval of 2.34-25.62 (p-value <0.001*)

in the second revision the regression model included two overlapping variables concerning smoking.

Cigarette Smoking: with a confidence interval of 11.57-73.612 (p-value <0.001*)

Cigarette Accessibility: with a confidence interval of 0.25-229.55 (p-value 0.241)

While Cigarette Smoking is significant in both the first and second revision, the second revision Cigarette Smoking has a much broader CI than the first revision while the overlapping variable Cigarette Accessibility has a massively large CI range 0.25-229.55. Likely co-linearity again.

Please rerun the regression model for Table 5 with either Cigarette Smoking or Cigarette Accessibility alone but not both. One could try each one separately for interest. My hunch is either will be significant and the CI range much narrower.

: We have re-run a final model again with removed cigarette accessibility out of the final model. The range of CI of cigarette smoking was narrower than 2nd revised. All information has been revised and improved accordingly. Please see in page.2, lines 3-13, page 22 lines 3-15, and table 5 in page 23-24.

Thank you,

TK

Tawatchai Apidehckul

Attachment

Submitted filename: MA_Reviewers_commenst_auresp_3.pdf

pone.0344775.s008.pdf (66.6KB, pdf)

Decision Letter 3

Ghaffar Ali

3 Feb 2026

Dear Dr. Apidechkul,

Please address the final minor comments from Reviewer 2.

Please submit your revised manuscript by Mar 18 2026 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org . When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

  • A letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

  • A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.

  • An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: https://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols . Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols .

We look forward to receiving your revised manuscript.

Kind regards,

Alejandro Torrado Pacheco, PhD

Associate Editor

PLOS One

on behalf of

Ghaffar Ali, PhD

Academic Editor

PLOS One

Journal Requirements:

If the reviewer comments include a recommendation to cite specific previously published works, please review and evaluate these publications to determine whether they are relevant and should be cited. There is no requirement to cite these works unless the editor has indicated otherwise.

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions??>

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously? -->?>

Reviewer #1: N/A

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available??>

The PLOS Data policy

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English??>

Reviewer #1: Yes

Reviewer #2: Yes

**********

Reviewer #1: Dear Authors,

I do not have further comments to add at this moment. However, it is appreciated that you have invested time to recheck the regression models, and the formatting of the paper, including the in-text citations and the references.

Thank you.

Reviewer #2: 1.27.2026 Comments on third revision.

Thank you again for re-running the model for Table 5 with one smoking variable. It is fine. I appreciate your long patience for all of my requests.

In closing I would like to suggest a substantive comment for Table 4 and a minor edit in introducing Table 5. Fielder's choice (meaning your choice whether to do this) ...

%%%%%%%%%%%%%%%%%%%%%%%%

** PAGE 17 concerning Table 4 results:

Looking at Table 4’s Univariate analysis on pages 20-21 I notice a pattern when comparing odds ratios between ‘friend culture’ and ‘family culture’ regarding Smoking, Alcohol use, Cannabis etc. use, MA use, and MA pressure. All ORs were significant, however the Friend ORs were systematically higher than the Family ORs for each of these:

Smoking ORs: Friends 5.04, Family 2.68

Alcohol use ORs: Friends 3.20, Family 3.05

Cannabis etc use ORs: Friends 4.74, Family 1.98

MA use ORs: Friends 11.26, Family 2.59

MA use pressure ORs: Friends 15.77, Family 4.24

So in every case ORs on the impact of Friends was higher than ORs on the impact of Family, sometimes much higher.

Given this, on page 17 after the last paragraph (17.14) you might add a comment such as:

“In the univariate analysis (Table 4) it is notable that the influence of friends exceeded that of the influence of family in the odds ratios in each category regarding Smoking, Alcohol use, Cannabis etc. use, MA use, and pressure to use MA – in some cases the Friends ORs were more than two- to three-fold higher than Family ORs . This has implications for prevention and intervention concerning social networks.”

[This comment also or instead could be placed somewhere in the Discussion section.]

%%%%%%%%%%%%%%%%%

** PAGE 22 introducing Table 5:

Page 22.4 model. => model (Table 5).

%%%%%%%%%%%%%%%%%%

I do not need to see the paper again. Thank you for doing this research. May it have fruitful effects for interventions for Lahu youth. Blessings!

**********

what does this mean? ). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy

Reviewer #1: No

Reviewer #2: Yes: Kirk C Allison

**********

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To ensure your figures meet our technical requirements, please review our figure guidelines: https://journals.plos.org/plosone/s/figures

You may also use PLOS’s free figure tool, NAAS, to help you prepare publication quality figures: https://journals.plos.org/plosone/s/figures#loc-tools-for-figure-preparation.

NAAS will assess whether your figures meet our technical requirements by comparing each figure against our figure specifications.

PLoS One. 2026 Mar 12;21(3):e0344775. doi: 10.1371/journal.pone.0344775.r008

Author response to Decision Letter 4


3 Feb 2026

Respond to reviewers’ comments

Dear Editor,

Thank you so much for the opportunity to revise this paper based on the valuable comments from the reviewers. We have carefully revised and do hope that you happy with this version.

Thank you,

TK

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Dear Authors,

I do not have further comments to add at this moment. However, it is appreciated that you have invested time to recheck the regression models, and the formatting of the paper, including the in-text citations and the references.

Thank you.

: Thank you very much

Reviewer #2: 1.27.2026 Comments on third revision.

Thank you again for re-running the model for Table 5 with one smoking variable. It is fine. I appreciate your long patience for all of my requests.

In closing I would like to suggest a substantive comment for Table 4 and a minor edit in introducing Table 5. Fielder's choice (meaning your choice whether to do this) ...

%%%%%%%%%%%%%%%%%%%%%%%%

** PAGE 17 concerning Table 4 results:

Looking at Table 4’s Univariate analysis on pages 20-21 I notice a pattern when comparing odds ratios between ‘friend culture’ and ‘family culture’ regarding Smoking, Alcohol use, Cannabis etc. use, MA use, and MA pressure. All ORs were significant, however the Friend ORs were systematically higher than the Family ORs for each of these:

Smoking ORs: Friends 5.04, Family 2.68

Alcohol use ORs: Friends 3.20, Family 3.05

Cannabis etc use ORs: Friends 4.74, Family 1.98

MA use ORs: Friends 11.26, Family 2.59

MA use pressure ORs: Friends 15.77, Family 4.24

So in every case ORs on the impact of Friends was higher than ORs on the impact of Family, sometimes much higher.

Given this, on page 17 after the last paragraph (17.14) you might add a comment such as:

“In the univariate analysis (Table 4) it is notable that the influence of friends exceeded that of the influence of family in the odds ratios in each category regarding Smoking, Alcohol use, Cannabis etc. use, MA use, and pressure to use MA – in some cases the Friends ORs were more than two- to three-fold higher than Family ORs . This has implications for prevention and intervention concerning social networks.”

: Thank you so much for your kind support. It’s added, please see page 17, line 15-19.

[This comment also or instead could be placed somewhere in the Discussion section.]

%%%%%%%%%%%%%%%%%

** PAGE 22 introducing Table 5:

Page 22.4 model. => model (Table 5).

: Thank you, it is added, page 22, line 4.

%%%%%%%%%%%%%%%%%%

I do not need to see the paper again. Thank you for doing this research. May it have fruitful effects for interventions for Lahu youth. Blessings!

: Thank you so much

TK

Decision Letter 4

Ghaffar Ali

25 Feb 2026

Factors influencing methamphetamine use among Lahu hill tribe youth in Chiang Rai, Thailand: A case-control study

PONE-D-25-27632R4

Dear Dr. Apidechkul,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

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Ghaffar Ali, PhD

Academic Editor

PLOS One

Additional Editor Comments (optional):

Reviewers' comments:

Acceptance letter

Ghaffar Ali

PONE-D-25-27632R4

PLOS One

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Associated Data

    This section collects any data citations, data availability statements, or supplementary materials included in this article.

    Supplementary Materials

    S1 Appendix. Question guide.

    (DOCX)

    pone.0344775.s001.docx (236.5KB, docx)
    S1 File. STROBE.

    (DOC)

    pone.0344775.s002.doc (81.5KB, doc)
    Attachment

    Submitted filename: ReviewReport_2025_PONE-D-25-27632.pdf

    pone.0344775.s003.pdf (3.9MB, pdf)
    Attachment

    Submitted filename: PONE-D-25-27632 review.docx

    pone.0344775.s004.docx (25.7KB, docx)
    Attachment

    Submitted filename: Respone to reviewer comments .pdf

    pone.0344775.s006.pdf (204.6KB, pdf)
    Attachment

    Submitted filename: MA_Reviewers commenst.pdf

    pone.0344775.s007.pdf (46.3KB, pdf)
    Attachment

    Submitted filename: MA_Reviewers_commenst_auresp_3.pdf

    pone.0344775.s008.pdf (66.6KB, pdf)

    Data Availability Statement

    Data cannot be shared publicly because it contains highly sensitive information. Data are available from the Center of Excellence for Hill Tribe Health Research, Mae Fah Luang Univeristy Institutional Data Access Committee https://cehr.mfu.ac.th/publications.html (cehr@mfu.ac.th) for researchers who meet the criteria for access to confidential data.


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