Abstract
This cohort study examines changes in the rates of solitary alcohol and marijuana use among 12th grade students between 1976 and 2019.
Recent reviews have highlighted adolescent solitary alcohol and marijuana use as risk indicators associated with negative consequences, coping motives, and negative affect1,2; solitary use may reflect self-medication.1,2 Adolescent solitary alcohol use is associated with health and academic problems,3 deviant behavior,3 and alcohol use disorder.4 Data on sex differences in solitary alcohol and marijuana use have been mixed.1,2 Nationally representative estimates of prevalence and change in adolescent solitary alcohol and marijuana use are needed.1 This study provides 2018-2019 prevalence estimates of and 1976-2019 trends in solitary alcohol and marijuana use among all 12th grade students and those who used alcohol and marijuana in the past 12 months, separated by sex.
Methods
Data were collected from spring 1976 through spring 2019 through the US nationally representative Monitoring the Future study.5 Informed consent (active or passive, per school policy) was obtained from parents (if students were <18 years) or students (if students were ≥18 years). A University of Michigan institutional review board approved the study. Mean student response rates were 82.4%.5 Solitary use was asked on 1 of 6 randomly distributed questionnaires. Respondents self-reported past 12-month alcohol and marijuana use and how often such use occurred when alone. For each substance, a dichotomous measure was coded, indicating any solitary use. Prevalence and SE estimates (calculating means of 2-year groupings to enhance stability) were obtained using survey procedures in SAS 9.4 (SAS Institute Inc), accounting for the complex sample design. Using obtained estimates, models6 estimated linear change over time using Joinpoint version 4.7.0.0 (National Cancer Institute); statistical significance was defined as P < .05 (2 sided).
Results
Of the 121 279 students responding to the relevant questionnaire, 9171 (7.6%) had missing data on sex; another 1482 (1.2%) had missing data on past-12-month alcohol and/or marijuana use or solitary use, leaving 110 626 responses. The sample included 57 257 female students (51.8%). Among all 12th grade students in 2018-2019, 14.8% (95% CI, 13.4%-16.3%) reported solitary alcohol use and 15.8% (95% CI, 14.2%-17.4%) reported solitary marijuana use. Among those who had used alcohol in the past 12 months in 2018-2019, solitary drinking was reported by 23.5% (95% CI, 20.4%-26.6%) of female students and 30.0% (95% CI, 26.1%-33.9%) of male students; percentages for solitary marijuana use among those who used marijuana were 42.3% (95% CI, 37.7%-47.0%) of female students and 54.8% (95% CI, 50.0%-59.6%) of male students.
Trend analyses (Figure; Table) showed among all students, solitary alcohol use decreased nonsignificantly from 1976-1977 through 1986-1987 (slope 1 [SE], −0.547 [0.254]; P = .05), showed a slope not significantly different than 0 through 1992-1993, and then decreased significantly through 2018-2019 (slope 3 [SE], −0.182 [0.060]; P = .009). Solitary marijuana use among all students decreased significantly from 1976-1977 through 1992-1993 (slope 1 [SE], −1.826 [0.136]; P < .001), then had no significant change through 1998-1999, and then increased significantly through 2018-2019 (slope 3 [SE], 0.362 [0.126]; P = .01). Among those who used alcohol in the past 12 months, solitary alcohol use decreased significantly from 1976-1977 through 2000-2001 for female students (slope 1 [SE], −0.644 [0.146]; P < . 001) and through 2014-2015 for male students (slope 1 [SE], −1.375 [0.165]; P < .001; slope 2 [SE], −0.425 [0.129]; P = .005). Among female students, prevalence increased significantly from 2000-2001 through 2018-2019 (slope 2 [SE], 0.832 [0.298]; P = .01). In contrast, from 2014-2015 through 2018-2019, there was no significant prevalence change among male students. The percentage of both female and male students who used marijuana reporting solitary marijuana use decreased significantly from 1976-1977 through 1992-1993 (female students: slope 1 [SE], −2.214 [0.280]; P < .001; male students: slope 1 [SE], −1.819 [0.306]; P < .001), and then increased significantly from 1992-1993 through 2018-2019 (female students: slope 2 [SE], 1.613 [0.174]; P < .001; male students: slope 2 [SE], 1.763 [0.182]; P < .001). Solitary marijuana use prevalence estimates among students reporting use in 2018-2019 were the highest level observed since data collection began in 1976.
Figure. Modeled Trends in Solitary Use of Alcohol and Marijuana in US 12th Grade Students, 1976-2019.
Data are reported using 2-year groupings. Numbers (unweighted) for alcohol: all students, 107 297; female users, 41 901; male users, 39 950; for marijuana: all students, 108 210; female users, 19 101; male users, 20 845. Trends were modeled using Joinpoint.
Table. Slope Estimates for Trends in Solitary Use of Alcohol and Marijuana in US 12th Grade Students, 1976-2019.
| Characteristic | No. (unweighted) | Slope 1 (SE) | P valuea | Joinpointb | Slope 2 (SE) | P value | Joinpoint | Slope 3 (SE) | P value |
|---|---|---|---|---|---|---|---|---|---|
| Among all students | |||||||||
| Solitary use | |||||||||
| Alcohol | 107 297 | −0.547 (0.254) | .05 | 1986-1987 | −2.363 (1.227) | .08 | 1992-1993 | −0.182 (0.060) | .009 |
| Marijuana | 108 210 | −1.826 (0.136) | <.001 | 1992-1993 | 1.836 (1.321) | .19 | 1998-1999 | 0.362 (0.126) | .01 |
| Among those reporting use in the past 12 mo | |||||||||
| Solitary alcohol use | |||||||||
| Female | 41 901 | −0.644 (0.146) | <.001 | 2000-2001 | 0.832 (0.298) | .01 | NA | NA | NA |
| Male | 39 950 | −1.375 (0.165) | <.001 | 1992-1993 | −0.425 (0.129) | .005 | 2014-2015 | 1.966 (2.187) | .38 |
| Solitary marijuana use | |||||||||
| Female | 19 101 | −2.214 (0.280) | <.001 | 1992-1993 | 1.613 (0.174) | <.001 | NA | NA | NA |
| Male | 20 845 | −1.819 (0.306) | <.001 | 1992-1993 | 1.763 (0.182) | <.001 | NA | NA | NA |
Abbreviation: NA, not applicable.
The P values of the slope changes are calculated from t tests based on asymptotic normality; the number of joinpoints is calculated using a permutation test procedure not requiring asymptotic normality and maintaining correct type I error probability.
Joinpoint or year grouping in which significant change in slope occurred.
Discussion
In 2018-2019, solitary drinking and solitary marijuana use were each reported by approximately 15% of all US 12th grade students. The percentage of female 12th grade students who had used alcohol in the past 12 months who reported solitary alcohol use is increasing, as are percentages of both male and female students who use marijuana and reported solitary marijuana use.
Limitations include the use of cross-sectional, self-reported data. Findings may not generalize to individuals who drop out of school. To the extent that solitary alcohol and marijuana use are indicators for significant risk of a range of negative outcomes,1,2 these data indicate growing cause for concern for a substantial percentage of adolescent substance users.
References
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