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Background States are rapidly moving to reverse marijuana prohibition, most frequently through legalization of medical marijuana laws (MMLs), and there is concern that marijuana legalization may affect adolescent marijuana use. Methods This natural-experimental study used state Youth Risk Behavior Survey (YRBS) data collected from participants in grades 9–12 from 1991 to 2015 in 46 states ( N = 1,091,723). Taking advantage of heterogeneity across states in MML status and MML dispensary design, difference-in-difference estimates compared states with enacted MMLs/dispensaries to non-MML/dispensaries states. Multivariable logistic regression modeling was used to adjust for state and year effects, and student demographics. The main outcome assessed was past 30-day adolescent marijuana use [“any” and “heavy” (≥20)]. Results In the overall sample, the adjusted odds of adolescents reporting any past 30-day marijuana use was lower in states that enacted MMLs at any time during the study period (OR 0.94, 95% CI 0.89 to 0.99; p < .05), and in states with operational dispensaries in 2015 (OR 0.93, 95% CI 0.88 to 0.99; p < .05). Among grade cohorts, only 9th graders showed a significant effect, with lower odds of use with MML enactment. We found no effects on heavy marijuana use. Conclusions This study found no evidence between 1991 and 2015 of increases in adolescents reporting past 30-day marijuana use or heavy marijuana use associated with state MML enactment or operational MML dispensaries. In a constantly evolving marijuana policy landscape, continued monitoring of adolescent marijuana use is important for assessing policy effects.

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sample of patients who made 2 or more purchases of a medical cannabis product. d This sample reflects data reported in the Medical Use of Marijuana Under the Compassionate Care Act Two Year Report. e Conditions are not mutually exclusive; therefore, percentages may total more than 100%. f Opioid replacement was not an approved qualifying condition for medical cannabis use during the study period. Author affiliations and article information are listed at the end of this article.