Grade Level Differences in the Association between Medical Marijuana Laws and Marijuana Use Among US High School Students, 1991-2015

Introduction: Prior research has not shown a significant association between medical marijuana laws (MMLs) and increases in adolescent marijuana use. However, concerns still exist, specifically surrounding specific more permissive MML provisions (e.g. medical marijuana dispensaries) and differential grade cohort effects. We re-evaluate the impact of MMLs on marijuana use among adolescents; our work: (1) uses recent data, (2) includes an assessment of state-level dispensary systems, a key MML provision that may increase access to marijuana for adolescents, and (3) differential grade cohort effects.

Methods: This quasi-experimental study uses data from the Youth Risk Behavior Survey, which includes data on 9th-12th graders from 1991-2015 from 46 states (N= 1,072,903). We assessed the effects of three factors (i.e., MMLs, whether dispensaries are permitted, and whether dispensaries were active by 2015) on past 30-day marijuana use. Nineteen states in the dataset enacted MMLs during study years, 1991-2015. Taking advantage of heterogeneity across states in MML dispensary status and design, difference-in-difference estimates compared states with enacted MMLs/dispensary provisions to non-MML/dispensary provisions states. Multiple logistic regression modeling was used to adjust for state and year effects, and key demographic factors

Results: Adjusted findings suggest that across all grades, past 30-day marijuana use post-MML was significantly lower among adolescents residing in states with MMLs compared with adolescents residing in non-MML states (aOR=0.91 (95% CI 0.87-0.96), p<.001); with significant lower odds found among 9th, 10th, and 11th graders (respectively, aOR=0.88 (95% CI 0.80-0.96), aOR=0.91 (95% CI 0.84-0.98), aOR=0.88 (95% CI 0.81-0.96). Similar findings were found in MML states that had active medical marijuana dispensaries by 2015 with significant lower odds found across all grades (i.e., OR=0.92 (95% CI 0.87-0.98) and specifically for 9th, 10th, and 11th graders, but not among 12th graders. Whether state MMLs permitted dispensaries did not show any significance for any grade cohort.

Conclusions: Consistent with existing literature, we found that adolescents residing in states with MMLs have lower odds of past 30-day marijuana use, particularly 9th-11th graders. This result did not change direction when assessing whether dispensaries were permitted or whether states had active medical marijuana dispensaries by 2015, a MML provision hypothesized to increase adolescent assess to and use of marijuana. Our results, with the most current and inclusive national data, presents similar findings of prior studies that have reported either no change or lower odds of adolescent marijuana use behaviors post MML-enactment.