Abstract
Objective:
Young adults who engage in simultaneous alcohol and cannabis use report heavy use of both substances. Event-level studies examining differences between consequences experienced on simultaneous use compared with single substance use days have been mixed. Although studies often control for alcohol use levels, few have examined how quantity of alcohol may influence consequences experienced on simultaneous use days. Furthermore, little research has examined the relationship between simultaneous use and positive consequences or explored individual consequences. This study examined differences in consequences experienced on simultaneous use versus single substance use days (alcohol-only, cannabis-only) including endorsement of specific consequences and examined the relationship among simultaneous use, consequences, and alcohol consumption.
Method:
Young adults who engage in simultaneous use (N = 86) completed 30 days of daily surveys reporting substance use and positive and negative consequences.
Results:
Simultaneous use days were associated with more negative and positive consequences than single substance use (alcohol-only and cannabis-only) days. We also examined endorsement of specific positive and negative consequences on alcohol-only, cannabis-only, and simultaneous use days. The effect of day type (simultaneous use vs. alcohol-only) on consequences was moderated by alcohol quantity. On lighter drinking days, more positive and negative consequences were endorsed if it was a simultaneous use versus alcohol-only day. On heavier drinking days, number of consequences did not differ between simultaneous use and alcohol-only days.
Conclusions:
Findings from this study point to simultaneous use as both reinforcing and high risk and highlight the importance of intervening even on lighter drinking simultaneous use occasions to reduce harms.
Many young adults report using both alcohol and cannabis concurrently (using both substances but not so they overlap) and/or simultaneously (using both substances so that their effects overlap; Brière et al., 2011; Subbaraman & Kerr, 2015; Terry-McElrath & Patrick, 2018), with simultaneous use more prevalent and associated with more negative outcomes (Subbaraman & Kerr, 2015). Although between-person studies have found that young adults who engage in simultaneous use report more consequences than those who only engage in single-substance use, results from event-level studies exploring the relationship between simultaneous use occasions and consequences have been mixed. Some studies show simultaneous (vs. single-substance) use occasions are associated with more negative consequences (Linden-Carmichael et al., 2020; Lipperman-Kreda et al., 2017), others show no difference (Lee et al., 2020; Mallett et al., 2019), and a few have found that the difference depends on which single substance is being compared with simultaneous use. For example, a recent study found that simultaneous use days were associated with more consequences than cannabis-only—but not alcohol-only—days (Sokolovsky et al., 2020).
Studies examining differences between simultaneous use and alcohol-only days suggest that quantity of alcohol may drive negative consequences experienced on simultaneous use days (Mallett et al., 2019; Sokolovsky et al., 2020). Day-level studies have found that simultaneous use occasions are associated with more alcohol consumption than alcohol-only occasions (Lee et al., 2020; Linden-Carmichael et al., 2020). Yet, it may be that alcohol quantity moderates the relationship between simultaneous use and consequences, such that differences in consequences experienced may be seen between alcohol-only and simultaneous use occasions at particular alcohol consumption levels. This has yet to be investigated.
Another gap in prior literature is that nearly all event-level studies examining the relationship between simultaneous use and consequences have examined consequences in aggregate. A notable exception is Lipperman-Kreda et al. (2017) who, in a sample of youth, found that compared with single-substance use, simultaneous use was associated with greater odds of violence, driving under the influence or riding with a drunk driver, and being drunk. Although this study tested the effect of simultaneous use on specific negative consequences, it only examined five broad consequence categories, two of which were alcohol specific. Mixed findings across studies may be explained by assessment of different consequences. Lack of a single comprehensive consequence scale may obscure differences in types of consequences experienced on simultaneous versus single substance use occasions. Further identification of specific consequences more likely to occur during simultaneous use, particularly in the young adult population for which simultaneous use is both normative and high risk, is needed.
Moreover, identifying the perceived positive effects reported during simultaneous use will further our understanding of how simultaneous use behavior is reinforced. Two daily survey studies have examined the relationship between simultaneous use and positive consequences (Lee et al., 2020; Linden-Carmichael et al., 2020), but findings were mixed as to whether simultaneous use is associated with a greater number of positive consequences than single-substance use. Again, this work failed to isolate specific individual consequences that may be more likely on simultaneous use occasions. Further understanding the relationship between simultaneous use and consequences (both positive and negative) will help characterize harm associated with simultaneous use and understand engagement and potential escalation of this high-risk behavior.
Present study
We conducted a daily diary study to further understand simultaneous use. Aim 1 was to test the hypothesis that simultaneous use days are associated with greater alcohol consumption and a greater likelihood of heavy episodic drinking (HED; 4+/5+ drinks for women/men) relative to alcohol-only days. Aim 2 was to test the hypothesis that simultaneous use days are associated with a greater number of both positive and negative consequences relative to single substance use (alcohol-only, cannabis-only) days. Although research to date has been mixed, we base this prediction on findings that days of heavier drinking tend to be associated with more positive and negative consequences (e.g., Barnett et al., 2014). In addition, some prior work has found simultaneous use to be associated with more consequences than cannabis-only use (Linden-Carmichael et al., 2020; Sokolovsky et al., 2020). We descriptively examined endorsement of specific positive and negative consequences on alcohol, cannabis, and simultaneous use days. We also explored which individual positive and negative consequences were more likely to occur on simultaneous use versus alcohol-only days. For Aim 3, we tested whether alcohol quantity moderates the relationship between simultaneous use and consequences experienced at the day level. Although we presumed that the day-level effect of simultaneous use on consequences may depend on how much alcohol was consumed that day, given the lack of prior research and/or theory related to this aim, we did not forward hypotheses about the precise nature of this potential interactive effect.
Method
Participants
Brown University Institutional Review Board approved all procedures. Participants were recruited nationally through social media and locally via flyers and university listservs. Eligibility criteria included being age 18–29, living in the United States, having used alcohol and cannabis so their effects overlapped (simultaneous use) at least twice in the past 30 days, at least weekly alcohol use, and access to a smartphone. Exclusion criteria included treatment in the past 6 months for substance use disorder or serious mental illness. A total of 657 individuals completed the screener, of which 124 were eligible. Eighty-nine participants (13.5% of those screened, 84% of those who provided contact information) were enrolled and completed 30 days of daily surveys; however, three participants did not report alcohol use during the 30 days and were removed from analyses. Thus, participants included 86 (60% female; Table 1) young adults.
Table 1.
Participant characteristics (N = 86)
| Variable | n (%) or M (SD) |
|---|---|
| Age, in years | 21.70 (2.47) |
| Biological sex | |
| Female | 49 (57%) |
| Gender (check all that apply) | |
| Woman | 49 (57%) |
| Man | 34 (40%) |
| Trans man | 2 (2%) |
| Genderqueer/gender nonconforming | 1 (1%) |
| Race (check all that apply) | |
| White | 57 (66%) |
| African American | 14 (16%) |
| Asian | 18 (21%) |
| Native American/Alaskan | 1 (1%) |
| Hawaiian/Pacific Islander | 1 (1%) |
| Other | 5 (6%) |
| Ethnicity | |
| Hispanic/Latino | 11 (13%) |
| College attendance | |
| Current student | 70 (81%) |
| Current state residence recreational cannabis legalization status | |
| Legal | 33 (38%) |
| Baseline substance use behavior | |
| No. of drinking days per week | 3.31 (1.53) |
| No. of cannabis use days per week | 2.09 (1.86) |
| No. of simultaneous use days per week | 1.37 (1.33) |
| No. of drinks per week | 10.90 (6.32) |
| AUDIT | 9.36 (4.67) |
| CUDIT-R | 8.38 (5.50) |
| Daily survey (30 days) descriptives | |
| No. of alcohol-only days | 5.06 (4.45) |
| No. of cannabis-only days | 3.98 (5.46) |
| No. of simultaneous use days | 2.92 (2.57) |
| No. of drinks per alcohol-only days | 3.76 (2.86) |
| No. of drinks per simultaneous use days | 4.78 (3.59) |
| No. of positive consequences per alcohol-only days | 3.46 (2.87) |
| No. of negative consequences per alcohol-only days | 1.95 (2.71) |
| No. of positive consequences per cannabis-only days | 3.71 (2.94) |
| No. of negative consequences per cannabis-only days | 1.07 (1.66) |
| No. of positive consequences per simultaneous use days | 4.88 (3.48) |
| No. of negative consequences per simultaneous use days | 2.73 (2.64) |
Notes: No. = number; AUDIT = Alcohol Use Disorders Identification Test; CUDIT-R = Cannabis Use Disorder Identification Test–Revised. College attendance includes both 2- and 4-year college enrollment.
Procedures
Enrollment and orientation. Interested individuals completed an online screening questionnaire and those who met eligibility criteria were contacted to confirm eligibility and schedule an orientation session via Zoom. Participants completed an online consent for a baseline survey before orientation. During an individual orientation session, study procedures were described and consent was obtained for the daily survey phase. Participants were trained in reporting standard drinks, cannabis use, and simultaneous use. Participants were directed to report simultaneous use (a) when using alcohol and cannabis at the same time/close in time and (b) regardless of timing of actual use, when the effects of both substances were perceived to overlap (i.e., using cannabis when still feeling effects of earlier alcohol consumption, or using alcohol when still feeling effects of earlier cannabis consumption). Participants received a $25 Amazon.com gift card for completing the baseline survey and orientation.
Daily survey protocol. The day following orientation, participants began 30 days of daily surveys. Morning reports were available from 7 A.M. to 5 P.M. and evening reports were available from 5 P.M. to 10 P.M. Participants received $1.25 for each completed survey, resulting in a $75 Amazon.com gift card for 100% adherence.
Baseline measures
Demographics. Demographics assessed included age, sex, gender, race, ethnicity, college attendance status, and current state residence.
Substance use behavior. Participants reported their alcohol use during a typical week in the past 30 days (Collins et al., 1985). Data were summarized as total drinks and number of drinking days per typical week. In addition, participants reported the number of cannabis and simultaneous use days in a typical week. Participants also completed the Alcohol Use Disorders Identification Test (Saunders et al., 1993) and the Cannabis Use Disorder Identification Test–Revised (Adamson et al., 2010).
Daily measures: Morning report
Alcohol, cannabis, and simultaneous use. Participants were asked, “Which types of substance use episodes did you have yesterday (check all that apply)?” (alcohol-only episode, cannabis-only episode, an episode where the effects of alcohol and cannabis overlapped, no alcohol or cannabis use). When participants reported prior-day alcohol use, follow-up questions assessed number of standard drinks consumed. A dichotomous indicator of whether each day was an HED day was created.
Nicotine and other drug use. Participants were asked, “Did you use any of the following while feeling the effects of cannabis/during your drinking episode?”: cigarettes, e-cigarettes/vape (nicotine), other tobacco products (e.g., smokeless, hookah, cigars), or other drugs. Separate indicators for nicotine and other drug use were created.
Daily measures: Evening report
Consequences. On days when participants reported prior-day alcohol, cannabis, or simultaneous use they were asked to report on 21 negative and 14 positive consequences. A list of consequences, designed to include those attributable to both alcohol and cannabis use, was derived from several measures including the Brief Young Adult Alcohol Consequences Questionnaire (Kahler et al., 2005), the Marijuana Problems Scale (Stephens et al., 2000), the Positive Drinking Consequences Questionnaire (Corbin et al., 2008), Marijuana Motives Questionnaire (Lee et al., 2009), Motives for Simultaneous Alcohol and Marijuana Use (Patrick et al., 2018), the Daily Alcohol-related Consequences and Evaluations for Young Adults Measure (Lee et al., 2017), the Acute Hangover Scale (Rohsenow et al., 2007), and our formative qualitative work (Boyle et al., 2023). Participants were asked, “During or after yesterday's drinking and/or cannabis use, which of the following experiences did you have (before you went to sleep)?” and participants checked off those experienced (1 = yes, 0 = no). Participants were also asked whether, after yesterday's drinking and/or cannabis use, they slept better than usual, slept worse than usual, or slept the same as usual. Responses were recoded into one negative consequence (1 = slept worse than usual, 0 = slept better or same as usual) and one positive consequence item (1 = slept better than usual and 0 = slept worse or same as usual). Participants were asked, “Rate your hangover today,” with response options from 0 (none) to 7 (incapacitating); responses were recoded into a dichotomous negative consequence item (0 = none, 1 = anything other than none). Also, participants were asked, “Check all of the following that apply to your memory for the time during or after yesterday's drinking and/or cannabis use.” Response options included period of time I cannot recall at all, some fuzzy memories, memories that become clear only after a reminder, and no difficulty remembering. This question was recoded into a dichotomous negative consequence variable (0 = no difficulty remembering, 1 = any type of memory impairment). Sum scores were created for total number of positive and negative consequences experienced each day.
Analytic plan
Hierarchical linear modeling (HLM), with event day (Level 1) nested within person (Level 2), was used to address our primary research questions examining whether day type (simultaneous vs. single-substance use) was a predictor of alcohol quantity and consequences.1 Models were run in the HLM 7.02 program (Raudenbush et al., 2013), using full information maximum likelihood estimation.
First, fully unconditional models (i.e., no predictors) were run to determine intraclass correlation coefficients (ICCs) for main outcomes. To examine differences in alcohol use during simultaneous use versus alcohol-only days, two models were run examining day type (simultaneous use day [coded 1] vs. alcohol-only day [coded 0]) as a predictor of (a) number of drinks consumed and (b) likelihood of drinking to an HED level (Aim 1). At Level 1, we controlled for (a) whether each day was a weekend (Friday/Saturday; coded 1) or weekday (Sunday–Thursday; coded 0) given increases in substance use on weekends and (b) day in study (1–30) to account for potential changes in drinking behavior over time. At Level 2, we controlled for sex (1 = male, 0 = female) and age. We also modeled a person-level indicator of the proportion of all drinking days that were simultaneous use days to examine effects of an individual's tendency to engage in simultaneous use on their level of alcohol use and isolate the unique influence of simultaneous use on alcohol quantity at the day level.
A second set of analyses was conducted to examine whether simultaneous use versus single substance use (alcohol-only, cannabis-only) days are associated with a greater number of (a) negative consequences and (b) positive consequences. Results examining number of consequences were presented using event rate ratios (ERRs), which estimate the change in the event rate of the outcome (number of consequences) for each one-unit increase in the independent variable. In addition, models examined whether simultaneous use versus alcohol-only days are associated with a greater likelihood of each of the 35 individual positive and negative consequences (Aim 2). In a third set of models, a Level 1 interaction between day type (simultaneous vs. alcohol-only) and alcohol quantity was included to test the hypothesis that amount of alcohol consumed moderates the relationship between simultaneous use and consequences (Aim 3). The interaction term was entered into each model alongside the main effect of each variable. These models examining consequences controlled at Level 1 for weekend/weekday, day in study, number of drinks consumed, whether nicotine was used, and whether other drugs were used because use of additional substances may increase positive and/or negative consequences experienced. At Level 2, we controlled for age and sex, and two person-level indicators representing (a) the proportion of all days that were simultaneous use days and (b) average drinks across all drinking days. For significant interactions, simple slopes were probed at heavier versus lighter drinking levels, 1 SD above and below one's own mean drinking level.
Results
Descriptives
Sample descriptives are presented in Table 1. Across 30 days, 2,397 (93%) morning reports and 2,348 (91%) evening reports were completed.2 The 86 participants reported 683 drinking days, of which 268 (39%) were HED and 251 (37%) were simultaneous use days. Additionally, 354 days were cannabis-only days. During the 30-day study period, 84% (n = 72) of participants reported at least one simultaneous use day (range: 1–15).3 Table 2 displays the number of participants who endorsed each consequence and number of times each consequence was endorsed on single substance use (alcohol-only, cannabis-only) and simultaneous use days. The ICC for number of total drinks consumed was .34, suggesting that 34% of the variability was between-person. The ICC was .24 for HED likelihood, .10 for number of positive consequences, and .22 for number of negative consequences. Across all outcomes, the majority of the variability was attributable to how participants differed from themselves across days.
Table 2.
Individual positive and negative consequences
| Consequences | n (% of 86 reported) | No. of simultaneous days (% of 251 days) | No. of alcohol-only days (% of 432 days) | No. of cannabis-only days (% of 354 days) |
|---|---|---|---|---|
| Negative consequences (21) | ||||
| Nausea or vomiting | 29 (34%) | 21 (8%) | 30 (7%) | 2 (1%) |
| Dizziness | 37 (43%) | 38 (15%) | 40 (9%) | 4 (1%) |
| Headache | 29 (34%) | 24 (10%) | 41 (9%) | 10 (3%) |
| Difficulty concentrating | 33 (38%) | 40 (16%) | 34 (8%) | 21 (6%) |
| Felt sleepy | 50 (58%) | 45 (18%) | 72 (17%) | 29 (8%) |
| Ate poorly or too much | 40 (47%) | 48 (19%) | 31 (7%) | 47 (13%) |
| Was rude or obnoxious | 13 (15%) | 5 (2%) | 13 (3%) | 2 (1%) |
| Hurt or injured self by accident | 12 (14%) | 8 (3%) | 8 (2%) | 0 (0%) |
| Verbally or physically aggressive | 5 (6%) | 3 (1%) | 2 (<1%) | 1 (<1%) |
| Embarrassed self | 32 (37%) | 20 (8%) | 32 (7%) | 6 (2%) |
| Interacted with someone in a way later regretted | 26 (30%) | 23 (9%) | 22 (5%) | 5 (1%) |
| Romantic/sexual experience later regretted | 9 (10%) | 7 (3%) | 9 (2%) | 0 (0%) |
| Spent more money than wanted to | 16 (19%) | 10 (4%) | 18 (4%) | 7 (2%) |
| Felt less social or avoided others | 21 (24%) | 13 (5%) | 21 (5%) | 7 (2%) |
| Felt anxious, worried, or paranoid | 32 (37%) | 27 (11%) | 29 (7%) | 27 (8%) |
| Felt down, depressed, or sad | 34 (40%) | 23 (9%) | 38 (9%) | 11 (3%) |
| Became fixated on something bothersome | 32 (37%) | 32 (13%) | 29 (7%) | 21 (6%) |
| Texted or used social media in a way later regretted | 21 (24%) | 13 (5%) | 17 (4%) | 2 (1%) |
| Slept worse than usual | 61 (71%) | 66 (26%) | 102 (24%) | 52 (15%) |
| Memory loss | 53 (62%) | 82 (33%) | 70 (16%) | 26 (7%) |
| Hangover | 74 (86%) | 135 (54%) | 160 (37%) | 42 (12%) |
| Positive consequences (14) | ||||
| Able to express feelings more easily | 54 (63%) | 46 (18%) | 71 (16%) | 47 (13%) |
| Felt more energetic | 60 (70%) | 75 (30%) | 76 (18%) | 53 (15%) |
| Mood improved | 71 (83%) | 103 (41%) | 111 (26%) | 116 (33%) |
| More sociable | 78 (91%) | 120 (48%) | 171 (40%) | 80 (23%) |
| Felt more confident | 65 (76%) | 79 (31%) | 88 (20%) | 44 (12%) |
| Felt relaxed | 78 (91%) | 142 (57%) | 175 (41%) | 211 (60%) |
| Forgot worries/problems | 54 (63%) | 83 (33%) | 70 (16%) | 84 (24%) |
| Shared a laugh with others | 80 (93%) | 149 (59%) | 212 (49%) | 137 (39%) |
| Bonded or felt closer to others | 72 (84%) | 112 (45%) | 153 (35%) | 93 (26%) |
| Positive romantic/sexual experience | 47 (55%) | 52 (21%) | 57 (13%) | 67 (19%) |
| Made a new friend or acquaintance | 51 (59%) | 62 (25%) | 69 (16%) | 14 (4%) |
| Had a really fun/exciting time | 75 (87%) | 113 (45%) | 136 (31%) | 74 (21%) |
| Had a creative moment/experience | 31 (36%) | 28 (11%) | 24 (6%) | 40 (11%) |
| Slept better than usual | 49 (57%) | 78 (31%) | 59 (14%) | 92 (26%) |
Note: No. = number.
Alcohol use on simultaneous use versus alcohol-only days
Results from the model predicting alcohol quantity indicated that more drinking occurred on simultaneous use days than on alcohol-only days. In addition, results from the model predicting HED likelihood indicated that HED was more likely on simultaneous use compared with alcohol-only days (Table 3).
Table 3.
Alcohol consumption on simultaneous use days compared with alcohol-only days
| Variable | Number of drinks | Likelihood of heavy episodic drinking (HED) | ||||
|---|---|---|---|---|---|---|
| B | SE | t | p | OR | [95% CI] | |
| Level 1 | ||||||
| Intercept | 3.32 | 0.30 | 11.08 | <.001 | 0.69 | [0.39, 1.20] |
| Simultaneous use (vs. alcohol only) | 0.84 | 0.25 | 3.40 | .001 | 2.04 | [1.25, 3.32] * |
| Weekend (vs. weekday) | -0.16 | 0.24 | -0.67 | .503 | 1.12 | [0.77, 1.63] |
| Study day (1–30) | 0.02 | 0.01 | 1.14 | .256 | 0.99 | [0.97, 1.01] |
| Level 2 | ||||||
| Age | -0.30 | 0.08 | -3.63 | <.001 | 0.79 | [0.69, 0.90]** |
| Male (vs. female) | 1.26 | 0.41 | 3.01 | .004 | 1.03 | [0.57, 1.87] |
| Mean simultaneous use days | -0.22 | 0.60 | -0.40 | .864 | 0.92 | [0.34, 2.55] |
Notes: OR = odds ratio; CI = confidence interval. Bold indicates the effects of interest when significant. Random slopes for study day, weekend, and day type for model predicting number of drinks. Day type is coded 1 = simultaneous use, 0 = alcohol-only use; sex is coded 0 = female, 1 = male; weekend is coded 0 = Sunday–Thursday, 1 = Friday or Saturday.
p < .01;
p < .001.
Consequences on simultaneous use versus alcohol-only days
Simultaneous use on a given day and higher alcohol quantity each were associated with more negative consequences (Table 4). This main effect was qualified by a significant interaction between day type (simultaneous vs. alcohol-only use) and alcohol quantity. Similarly, simultaneous use on a given day and higher alcohol quantity were each associated with more positive consequences. These main effects were again qualified by a significant interaction between day type and alcohol quantity.
Table 4.
Number of positive and negative consequences on simultaneous use days compared with alcohol-only days
| Variable | No. of negative consequences | No. of positive consequences | ||
|---|---|---|---|---|
| ERR | [95% CI] | ERR | [95% CI] | |
| Level 1 | ||||
| Intercept | 1.82 | [1.33, 2.49]** | 3.44 | [2.91, 4.08]** |
| Simultaneous use (vs. alcohol only) | 1.38 | [1.12, 1.70] ** | 1.32 | [1.16, 1.51] ** |
| Simultaneous Use × Drinks | 0.90 | [0.82, 0.99] * | 0.90 | [0.86, 0.95] ** |
| Weekend (vs. weekday) | 1.15 | [1.00, 1.33]* | 1.02 | [0.92, 1.12] |
| Study day (1–30) | 0.99 | [0.98, 1.00] | 1.00 | [0.99, 1.01] |
| Total drinks | 1.18 | [1.07, 1.31]** | 1.14 | [1.09, 1.19]** |
| Nicotine use (vs. none) | 1.31 | [1.00, 1.70]* | 1.00 | [0.81, 1.24] |
| Other drug use (vs. none) | 1.17 | [0.84, 1.63] | 1.16 | [0.99, 1.38] |
| Level 2 | ||||
| Age | 0.98 | [0.92, 1.05] | 0.92 | [0.87, 0.98]** |
| Male (vs. female) | 0.71 | [0.49, 1.04] | 1.06 | [0.83, 1.35] |
| Mean simultaneous use days | 1.03 | [0.59, 1.78] | 1.23 | [0.90, 1.71] |
| Average drinks | 1.16 | [1.05, 1.28]** | 0.97 | [0.91, 1.03] |
Notes: No. = number; ERR = event rate ratio; CI = confidence interval. Bold indicates the effects of interest when significant. Day type is coded 1 = simultaneous use, 0 = alcohol-only use; sex is coded 0 = female, 1 = male; weekend is coded 0 = Sunday–Thursday, 1 = Friday or Saturday; nicotine use is coded 1 = yes, 0 = no; other drug use is coded 1 = yes, 0 = no.
p < .01;
p < .001.
We probed the significant interactions by examining the effect of day type at heavier (relative to one's average) versus lighter (relative to one's average) alcohol quantity, where heavier and lighter values of drinking were calculated using each person's own mean and standard deviation across days. Mean total drinks for participants ranged from 1 to 9.82 drinks and standard deviations ranged from 0 to 6.14. The average lighter drinking day for the sample was 1.91 drinks and the average heavier drinking day for the sample was 6.39 drinks. On lighter drinking days, more positive (ERR = 1.83, 95% CI [1.49, 2.25]) and negative (ERR = 1.91, 95% CI [1.22, 3.00]) consequences were reported if it was a simultaneous use versus alcohol-only day. On heavier drinking days, number of positive (ERR = 0.95, 95% CI [0.80, 1.15]) and negative (ERR = 0.99, 95% CI [0.77, 1.27]) consequences did not differ between simultaneous use and alcohol-only days (Figure 1). In other words, the addition of cannabis use on drinking days increased positive and negative consequences only when drinking was lighter rather than heavier than usual.
Figure 1.
Drinking level moderates the relationship between simultaneous use and number of (A) negative consequences and (B) positive consequences. Notes: Drinking levels were calculated using each person's own mean and standard deviation across days. Heavier drinking is 1 SD above one's own mean drinking level and lighter drinking is 1 SD below one's own mean drinking level.
Using a Bonferroni correction and setting the p value at <.0014 (p value of .05 divided by 35 comparisons), a significant interaction between day type and alcohol quantity was found for the following negative consequences: dizziness; headache; difficulty concentrating; and feeling anxious, worried, or paranoid. No significant interaction between day type and alcohol quantity was found for individual positive consequences (Table 5).
Table 5.
Examining whether alcohol consumption moderates the relationship between simultaneous use and consequences
| Consequences | Simultaneous vs. alcohol-only use OR [95 % CI] | Day Type × Drinks OR [95 % CI] | Simple slopes – Heavier drinking days OR [95 % CI] | Simple slopes – Lighter drinking days OR [95 % CI] |
|---|---|---|---|---|
| Negative consequences (21) | ||||
| Nausea or vomitinga | 0.97 [0.36, 2.60] | – | – | – |
| Dizziness | 1.99 [0.95, 4.17] | 0.67 [0.55, 0.83]*** | 0.57 [0.22, 1.46] | 6.92 [2.42, 19.81]*** |
| Headache | 1.79 [0.74, 4.30] | 0.75 [0.61, 0.93]*** | 0.73 [0.32, 1.67] | 4.40 [1.17, 16.62]* |
| Difficulty concentrating | 3.32 [1.40. 7.88]*** | 0.61 [0.49, 0.76]*** | 0.69 [0.27, 1.77] | 15.94 [4.46, 57.00]*** |
| Felt sleepy | 1.16 [0.61, 2.22] | – | – | – |
| Ate poorly or too much | 3.82 [1.81, 8.06]*** | – | – | – |
| Was rude or obnoxious | 0.84 [0.16, 4.31] | – | – | – |
| Hurt or injured self by accidenta | 2.27 [0.63, 8.14] | 0.66 [0.45, 0.98]* | 0.61 [0.15, 2.51] | 8.32 [1.01, 68.30]* |
| Verbally or physically aggressivea,b | 1.20 [0.15, 10.03] | – | – | – |
| Embarrassed selfa | 0.61 [0.30, 1.27] | 0.72 [0.55, 0.95]* | 0.22 [0.08, 0.60]** | 1.72 [0.50, 5.97] |
| Interacted with someone in a way later regretteda | 1.13 [0.43, 2.98] | – | – | – |
| Romantic/sexual experience later regretteda | 1.60 [0.37, 6.87] | – | – | – |
| Spent more money than wanted toa | 0.57 [0.15, 2.13] | – | – | – |
| Felt less social or avoided othersa | 0.59 [0.23, 1.48] | – | – | – |
| Felt anxious, worried, or paranoid | 2.16 [1.01, 4.62]* | 0.69 [0.55, 0.86]*** | 0.65 [0.24, 1.75] | 7.16 [2.41, 21.24]*** |
| Felt down, depressed, or sad | 1.40 [0.68, 2.88] | 0.82 [0.71, 0.93]* | 0.73 [0.32, 1.66] | 2.59 [1.24, 5.42]* |
| Became fixated on something bothersome | 3.06 [1.60, 5.86]*** | – | – | – |
| Texted or used social media in a way later regretteda | 1.16 [0.61, 2.21] | – | – | – |
| Slept worse than usual | 0.94 [0.54, 1.63] | – | – | – |
| Memory loss | 2.13 [1.15, 3.95]* | – | – | – |
| Hangover | 1.54 [0.90, 2.64] | 0.73 [0.57, 0.95]* | 0.58 [0.20, 1.68] | 4.10 (1.75, 9.63)** |
| Positive consequences (14) | ||||
| Able to express feelings more easily | 1.16 [0.54, 2.47] | – | – | – |
| Felt more energetic | 1.58 [0.85, 2.93] | 0.83 [0.69, 0.99]* | 0.86 [0.36, 2.08] | 2.89 [1.26, 6.65]* |
| Mood improved | 1.91 [1.12, 3.24]* | – | – | – |
| More sociable | 1.31 [0.87, 1.98] | – | – | – |
| Felt more confident | 0.96 [0.54, 1.74] | – | – | – |
| Felt relaxed | 2.26 [1.29, 3.96]** | 0.77 [0.65, 0.92]** | 1.00 [0.48, 2.11] | 5.11 [2.29, 11.40]*** |
| Forgot worries/problems | 2.61 [1.44, 4.72]** | 0.77 [0.63, 0.95]* | 1.14 [0.53, 2.45] | 5.99 [2.22, 16.18]*** |
| Shared a laugh with others | 1.61 [1.06, 2.46]* | 0.81 [0.70, 0.94]** | 0.83 [0.47, 1.45] | 3.16 [1.56, 6.37]** |
| Bonded or felt closer to others | 1.54 [1.01, 2.37]* | 0.79 [0.67, 0.95]* | 0.75 [0.37, 1.53] | 3.20 [1.59, 6.41]** |
| Positive romantic/sexual experiencea | 1.25 [0.66, 2.37] | – | – | – |
| Made a new friend or acquaintance | 1.39 [0.77, 2.50] | – | – | – |
| Had a really fun/exciting time | 1.79 [1.96, 3.33]* | – | – | – |
| Had a creative moment/experiencea | 2.55 [1.09, 5.99]* | 0.72 [0.58, 0.89]** | 0.89 [0.34, 2.34] | 7.31 [2.16, 24.81]*** |
| Slept better than usuala | 1.93 [0.97, 3.85] | – | – | – |
Notes: OR = odds ratio; CI = confidence interval.
Model did not control for other drug use because other drug use never occurred on days when this consequence was reported (i.e., no variability in this predictor);
model did not control for nicotine use because nicotine use never occurred on days when this consequence was reported (i.e., no variability in this predictor).
p < .05;
p < .01;
p < .001.
Probing simple slopes at heavier (relative to one's average) and lighter (relative to one's average) alcohol quantity, we found that on lighter drinking days each of these negative consequences were more likely to be experienced if it was a simultaneous use versus alcohol-only day. On heavier drinking days, these same individual consequences did not differ between simultaneous use and alcohol-only days (Table 5). In single consequence models where no interaction was observed, we interpreted main effects of day type. Simultaneous use compared with alcohol-only days were associated with a greater likelihood of two negative consequences: ate poorly or too much and became fixated on something bothersome (Table 5).
Consequences on simultaneous use versus cannabis-only days
Simultaneous use on a given day was associated with more negative consequences relative to cannabis-only use. Similarly, simultaneous use was associated with more positive consequences relative to cannabis-only use (Table 6).
Table 6.
Number of positive and negative consequences on simultaneous use days compared with cannabis-only days
| Variable | No. of negative consequences | No. of positive consequences | ||
|---|---|---|---|---|
| ERR | [95% CI] | ERR | [95% CI] | |
| Level 1 | ||||
| Intercept | 1.28 | [0.89, 1.83] | 3.19 | [2.56, 3.97]** |
| Simultaneous use (vs. cannabis only) | 1.98 | [1.53, 2.55] ** | 1.39 | [1.20, 1.62] ** |
| Weekend (vs. weekday) | 0.87 | [0.75, 1.01] | 1.05 | [0.94, 1.18] |
| Study day (1–30) | 0.99 | [0.98, 1.01] | 1.00 | [0.99, 1.01] |
| Level 2 | ||||
| Age | 0.99 | [0.91, 1.83] | 0.94 | [0.88, 1.01] |
| Male (vs. female) | 1.07 | [0.69, 1.68] | 0.97 | [0.74, 1.28] |
| Mean simultaneous use days | 0.63 | [0.32, 1.23] | 1.13 | [0.74, 1.74] |
Notes: No. = number; ERR = event rate ratio; CI = confidence interval. Bold indicates the effects of interest when significant. Nicotine use and other drug use were not included in the model because they rarely occurred on cannabis-only days (i.e., no variability in this predictor). Day type is coded 1 = simultaneous use, 0 = cannabis-only use; sex is coded 0 = female, 1 = male; weekend is coded 0 = Sunday–Thursday, 1 = Friday or Saturday.
p < .001.
Discussion
This study found an increased number of positive and negative consequences on simultaneous use days compared with single substance use (both alcohol-only and cannabis-only) days. An examination of alcohol consumption revealed that participants consume more alcohol and are more likely to engage in HED on simultaneous use days compared with alcohol-only days, and alcohol quantity moderated the association between day-level simultaneous use and consequences. Findings suggest that cannabis may add another layer of both risk and reinforcement when drinking is lighter, but not heavier. Controlled studies examining the interactive effects of alcohol and cannabis at low blood alcohol concentrations have suggested additive or possibly synergetic effects from simultaneous use (Bramness et al., 2010; Liguori et al., 2002; Lukas & Orozco, 2001; Perez Reyes et al., 1988), which may lead to more positive and negative consequences at lighter drinking levels as compared with alcohol-only occasions.
An exploratory set of analyses in this study examined the association between simultaneous use and individual positive and negative consequences. Two negative consequences were more likely to be endorsed on simultaneous use versus alcohol-only days: ate poorly or too much and became fixated on something bothersome. Additionally, on lighter drinking days, certain cognitive and emotional consequences were more likely on simultaneous use versus alcohol-only days (dizziness; difficulty concentrating; feeling anxious, worried, or paranoid). Some of the specific consequences found to be more likely to be experienced on simultaneous use versus alcohol-only days, such as ate poorly or too much and feeling anxious, worried, or paranoid, are consequences known to be associated with cannabis use (Karila et al., 2014). Future work should examine differences in consequences experienced on simultaneous use versus cannabis-only days in order to further understand unique reinforcement and harms associated with simultaneous use. Importantly, the likelihood of experiencing most consequences assessed in this study did not differ on simultaneous use versus alcohol-only days. These findings correspond to our recent qualitative findings in which young adults reported numerous positive and negative consequences of simultaneous use, many of which overlapped with single-substance use (Boyle et al., 2023). However, young adults in this qualitative study perceived specific positive consequences to be enhanced and specific negative consequences to be more severe from simultaneous use compared with single-substance use, suggesting that it may not be differences in consequences experienced, but instead the severity of certain consequences, that differentiate outcomes on simultaneous use versus single substance use days. Thus, future quantitative work should examine the severity of consequences experienced in addition to endorsement of specific consequences. It is important to focus our attention on specific acute consequences to (a) better understand differential risk and reinforcement between single-substance use and simultaneous use and (b) provide formative work for the development of a simultaneous use consequence measure, given that much of the simultaneous use research has focused on alcohol-related consequences (Lipperman-Kreda et al., 2017; Sokolovsky et al., 2020), and differing consequence scales limit comparison between studies.
Continued exploration of patterns of use (i.e., quantity of substances, order, timing, and spacing of substances) as well as factors that influence use (i.e., motives, context) in relation to simultaneous use outcomes will help provide more insight as to when simultaneous use is most harmful and reinforcing. Beyond consideration of alcohol quantity during a simultaneous use occasion, aspects of the cannabis used could be explored. Yet, unlike alcohol quantity, which can be accurately assessed using measures of standard drinks, there is not a standardized unit of cannabis quantity that can be easily self-reported across various cannabis products. Controlled laboratory administration of alcohol at varying doses and cannabis at varying cannabinoid compositions and quantities could be conducted to examine acute effects of various combinations of alcohol and cannabis use. Such laboratory research could be integrated with ecological momentary assessment to examine simultaneous use in the real world (i.e., having young adults report alcohol and cannabis use, their current context, and the positive and negative outcomes experienced in real time) and help elucidate associations among patterns of use, event-level contextual factors, and outcomes in daily life.
The variability in use and consequences between single substance use and simultaneous use days highlights the importance of intervening at the day level. Ecological momentary intervention methods could deliver intervention content reminding participants of the negative consequences that can occur when consuming cannabis at a time during the day where alcohol use is still relatively moderate. In addition, specific consequences may provide concrete intervention targets. Learning which negative consequences are common or more severe during simultaneous use and which consequences predict short-term decisions to abstain from mixing alcohol and cannabis should be cultivated and emphasized in interventions. Identification of positive consequences that reinforce and predict future simultaneous use should be undermined and alternative reinforcers be promoted.
In addition to intervention implications noted above, understanding the extent to which cannabis influences alcohol use among young adults may have important policy implications, as more states debate recreational cannabis legalization in the United States. Findings that simultaneous use is associated with increased alcohol use may elicit reservations about adopting less restrictive policies that permit simultaneous use (and increased use of alcohol when cannabis is used). For example, states considering cannabis social consumption sites should consider the extent to which patrons may be intoxicated on entry. Future research examining simultaneous use patterns and related consequences should include comparisons of states with and without legalized recreational cannabis use. Examining simultaneous use across states over time will provide insight into how the changing legal and social landscape influence simultaneous use.
Limitations
To ensure sufficient base rates of alcohol and cannabis use across the study, the study focused on a specific group of young adults who reported drinking on a weekly basis and engaging in simultaneous use bi-weekly, on average. Young adults were not required to report a certain frequency of cannabis use, yielding a sample that reported more alcohol than cannabis use. In addition, the majority of the sample reported currently being enrolled in college. Therefore, findings may not generalize to non–college attending young adults who may exhibit different substance use patterns. In particular, future work should examine patterns of use and differences in consequences for simultaneous use versus single-substance use among those who engage in more frequent cannabis use. Those engaging in more frequent cannabis use may be more likely to experience different consequences compared with this study sample. Despite oversampling of a high-risk population, many of the negative consequences examined had low endorsement. Although certain consequences were significantly more likely to be reported on simultaneous use days, some of the confidence intervals were quite wide, suggesting greater uncertainty in findings. Future studies should examine larger samples in which individual consequences are more highly endorsed, or similar studies could be combined via integrative data analysis (Curran & Hussong, 2009) to examine more occasions of individual consequences. Also, more than 60% of participants lived in a state where recreational cannabis use was illegal at the time; the impact of the broader social–legal context on consequences deserves further exploration.
Conclusions
Study findings suggest the need to focus on multi-substance use among young adults to reduce harms. Teaching youth to avoid mixing alcohol and cannabis has been a recommended protective behavioral strategy (Treloar et al., 2015), and our findings suggest even greater benefit on lighter drinking days. Interventions (a) highlighting and informing young adults about the negative consequences of simultaneous use and (b) encouraging alternative activities/reinforcers may be important for minimizing this pattern of use.
Footnotes
This study was supported by a Research Society on Alcohol Doctoral Student Small Grant (principal investigator: Holly K. Boyle) and an American Psychological Association Division 50 Student Grant (principal investigator: Holly K. Boyle). Training support was also provided via National Institute on Alcohol Abuse and Alcoholism Grants F31AA028707 (principal investigator: Holly K. Boyle) and T32AA007459 (principal investigator: Robert Miranda, Jr.). Some of the data in this manuscript were previously presented as a poster at the 45th Annual Research Society on Alcohol, June 2022.
Because there were only 12 days of concurrent use of alcohol and cannabis (alcohol and cannabis used so their effects do not overlap), we restricted our analyses to simultaneous use days, alcohol-only days, and cannabis-only days.
Exploration of the data revealed an unrealistic response of 54 drinks consumed during one day. The response was recoded to 23, one unit greater than the next largest value (22).
All participants who reported either alcohol or cannabis use were included in the study because (a) they met the simultaneous use eligibility criteria during screening and (b) their data was still able to contribute to our understanding of differences in alcohol use and consequences reported across substance use days for those who report frequent simultaneous use.
References
- Adamson S. J., Kay-Lambkin F. J., Baker A. L., Lewin T. J., Thornton L., Kelly B. J., Sellman J. D. An improved brief measure of cannabis misuse: The Cannabis Use Disorders Identification Test–Revised (CUDIT-R) Drug and Alcohol Dependence. 2010;110(1-2):137–143. doi: 10.1016/j.drugalcdep.2010.02.017. [DOI] [PubMed] [Google Scholar]
- Barnett N. P., Clerkin E. M., Wood M., Monti P. M., O’Leary Tevyaw T., Corriveau D., Fingeret A., Kahler C. W. Description and predictors of positive and negative alcohol-related consequences in the first year of college. Journal of Studies on Alcohol and Drugs. 2014;75(1):103–114. doi: 10.15288/jsad.2014.75.103. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Brière F. N., Fallu J. S., Descheneaux A., Janosz M. Predictors and consequences of simultaneous alcohol and cannabis use in adolescents. Addictive Behaviors. 2011;36(7):785–788. doi: 10.1016/j.addbeh.2011.02.012. [DOI] [PubMed] [Google Scholar]
- Boyle H. K., Singh S., López G., Carey K. B., Jackson K. M., Merrill J. E. A qualitative examination of positive and negative consequences young adults experience from simultaneous alcohol and cannabis use. Psychology of Addictive Behaviors. 2023;37(4):639–650. doi: 10.1037/adb0000886. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Bramness J. G., Khiabani H. Z., Mørland J. Impairment due to cannabis and ethanol: Clinical signs and additive effects. Addiction. 2010;105(6):1080–1087. doi: 10.1111/j.1360-0443.2010.02911.x. [DOI] [PubMed] [Google Scholar]
- Collins R. L., Parks G. A., Marlatt G. A. Social determinants of alcohol consumption: The effects of social interaction and model status on the self-administration of alcohol. Journal of Consulting and Clinical Psychology. 1985;53(2):189–200. doi: 10.1037/0022-006X.53.2.189. [DOI] [PubMed] [Google Scholar]
- Corbin W. R., Morean M. E., Benedict D. The Positive Drinking Consequences Questionnaire (PDCQ): Validation of a new assessment tool. Addictive Behaviors. 2008;33(1):54–68. doi: 10.1016/j.addbeh.2007.06.003. [DOI] [PubMed] [Google Scholar]
- Curran P. J., Hussong A. M. Integrative data analysis: The simultaneous analysis of multiple data sets. Psychological Methods. 2009;14(2):81–100. doi: 10.1037/a0015914. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Kahler C. W., Strong D. R., Read J. P. Toward efficient and comprehensive measurement of the alcohol problems continuum in college students: The Brief Young Adult Alcohol Consequences Questionnaire. Alcoholism: Clinical and Experimental Research. 2005;29(7):1180–1189. doi: 10.1097/01.ALC.0000171940.95813.A5. [DOI] [PubMed] [Google Scholar]
- Karila L., Roux P., Rolland B., Benyamina A., Reynaud M., Aubin H. J., Lançon C. Acute and long-term effects of cannabis use: A review. Current Pharmaceutical Design. 2014;20(25):4112–4118. doi: 10.2174/13816128113199990620. [DOI] [PubMed] [Google Scholar]
- Lee C. M., Cronce J. M., Baldwin S. A., Fairlie A. M., Atkins D. C., Patrick M. E., Zimmerman L., Larimer M. E., Leigh B. C. Psychometric analysis and validity of the daily alcohol-related consequences and evaluations measure for young adults. Psychological Assessment. 2017;29(3):253–263. doi: 10.1037/pas0000320. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Lee C. M., Neighbors C., Hendershot C. S., Grossbard J. R. Development and preliminary validation of a comprehensive marijuana motives questionnaire. Journal of Studies on Alcohol and Drugs. 2009;70(2):279–287. doi: 10.15288/jsad.2009.70.279. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Lee C. M., Patrick M. E., Fleming C. B., Cadigan J. M., Abdallah D. A., Fairlie A. M., Larimer M. E. A daily study comparing alcohol related positive and negative consequences for days with only alcohol use versus days with simultaneous alcohol and marijuana use in a community sample of young adults. Alcoholism: Clinical and Experimental Research. 2020;44(3):689–696. doi: 10.1111/acer.14279. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Liguori A., Gatto C. P., Jarrett D. B. Separate and combined effects of marijuana and alcohol on mood, equilibrium and simulated driving. Psychopharmacology. 2002;163(3-4):399–405. doi: 10.1007/s00213-002-1124-0. [DOI] [PubMed] [Google Scholar]
- Linden-Carmichael A. N., Van Doren N., Masters L. D., Lanza S. T. Simultaneous alcohol and marijuana use in daily life: Implications for level of use, subjective intoxication, and positive and negative consequences. Psychology of Addictive Behaviors. 2020;34(3):447–453. doi: 10.1037/adb0000556. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Lipperman-Kreda S., Gruenewald P. J., Grube J. W., Bersamin M. Adolescents, alcohol, and marijuana: Context characteristics and problems associated with simultaneous use. Drug and Alcohol Dependence. 2017;179:55–60. doi: 10.1016/j.drugalcdep.2017.06.023. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Lukas S. E., Orozco S. Ethanol increases plasma (9)-tetrahydrocannabinol (THC) levels and subjective effects after marihuana smoking in human volunteers. Drug and Alcohol Dependence. 2001;64(2):143–149. doi: 10.1016/S0376-8716(01)00118-1. [DOI] [PubMed] [Google Scholar]
- Mallett K. A., Turrisi R., Trager B. M., Sell N., Linden-Carmichael A. N. An examination of consequences among college student drinkers on occasions involving alcohol-only, marijuana-only, or combined alcohol and marijuana use. Psychology of Addictive Behaviors. 2019;33(3):331–336. doi: 10.1037/adb0000458. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Patrick M. E., Fairlie A. M., Lee C. M. Motives for simultaneous alcohol and marijuana use among young adults. Addictive Behaviors. 2018;76:363–369. doi: 10.1016/j.addbeh.2017.08.027. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Perez-Reyes M., Hicks R. E., Bumberry J., Jeffcoat A. R., Cook C. E. Interaction between marihuana and ethanol: Effects on psychomotor performance. Alcoholism: Clinical and Experimental Research. 1988;12(2):268–276. doi: 10.1111/j.1530-0277.1988.tb00193.x. [DOI] [PubMed] [Google Scholar]
- Raudenbush S. W., Bryk A. S., Congdon R. HLM 7.01 for Windows [Computer software] Scientific Software International, Inc; 2013. [Google Scholar]
- Rohsenow D. J., Howland J., Minsky S. J., Greece J., Almeida A., Roehrs T. A. The Acute Hangover Scale: A new measure of immediate hangover symptoms. Addictive Behaviors. 2007;32(6):1314–1320. doi: 10.1016/j.addbeh.2006.10.001. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Saunders J. B., Aasland O. G., Babor T. F., de la Fuente J. R., Grant M. Development of the alcohol use disorders identification test (AUDIT): WHO collaborative project on early detection of persons with harmful alcohol consumption II. Addiction. 1993;88(6):791–804. doi: 10.1111/j.1360-0443.1993.tb02093.x. [DOI] [PubMed] [Google Scholar]
- Sokolovsky A. W., Gunn R. L., Micalizzi L., White H. R., Jackson K. M. Alcohol and marijuana co-use: Consequences, subjective intoxication, and the operationalization of simultaneous use. Drug and Alcohol Dependence. 2020;212:107986. doi: 10.1016/j.drugalcdep.2020.107986. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Stephens R. S., Roffman R. A., Curtin L. Comparison of extended versus brief treatments for marijuana use. Journal of Consulting and Clinical Psychology. 2000;68(5):898–908. doi: 10.1037/0022-006X.68.5.898. [DOI] [PubMed] [Google Scholar]
- Subbaraman M. S., Kerr W. C. Simultaneous versus concurrent use of alcohol and cannabis in the National Alcohol Survey. Alcoholism: Clinical and Experimental Research. 2015;39(5):872–879. doi: 10.1111/acer.12698. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Terry-McElrath Y. M., Patrick M. E. Simultaneous alcohol and marijuana use among young adult drinkers: Age specific changes in prevalence from 1977 to 2016. Alcoholism: Clinical and Experimental Research. 2018;42(11):2224–2233. doi: 10.1111/acer.13879. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Treloar H., Martens M. P., McCarthy D. M. The protective behavioral strategies scale-20: Improved content validity of the serious harm reduction subscale. Psychological Assessment. 2015;27(1):340–346. doi: 10.1037/pas0000071. [DOI] [PMC free article] [PubMed] [Google Scholar]