Medical marijuana laws and adolescent marijuana use in the USA from 1991 to 2014: results from annual, repeated cross-sectional surveys

Abstract

Background Adolescent marijuana use is associated with adverse later-life consequences, so identifying factors underlying adolescent use is of substantial public health importance. The relationship of U.S. state medical marijuana laws (MML) to adolescent marijuana use has been controversial. Such laws could convey a message about marijuana acceptability that increases marijuana use soon after passage, even if implementation is delayed or the law narrowly limits use. We used 24 years of U.S. national data to examine the relationship between state MML and adolescent marijuana use. Methods Data came from 1,098,270 U.S. adolescents in 8th, 10th, and 12th grade in the national Monitoring the Future annual surveys conducted between 1991–2014. The main outcome was any marijuana use in the prior 30 days. Using multilevel regression modeling, we examined marijuana use in adolescents nested within states, including whether marijuana use was higher overall in states that ever passed a MML up to 2014, and whether the risk of use changed after state MML were passed. Individual-, school- and state-level covariates were controlled. Findings Overall, marijuana use was more prevalent in states that enacted MML up to 2014 than in other states (AOR=1.27, 95%CI=1.07–1.51). Pre- and post-MML risk did not differ in the full sample (AOR=0.92, 95%CI=0.82–1.04). A significant interaction (p<0.001) indicated differential post-MML risk by grade. In 8th graders, post-MML use decreased (AOR=0.73, 95%CI=0.63–0.84), while no significant change occurred in 10th or 12th graders. Results were generally robust across sensitivity analyses. Interpretation Previous evidence and this study show that MML passage does not result in increased adolescent marijuana use. However, overall, adolescent use is higher in states that ever enacted MML than in other states. State-level risk factors other than MML may contribute to both marijuana use and MML, warranting investigation. An observed 8th-grade post-MML decrease also merits further study. Funding U.S. National Institute on Drug Abuse, Columbia University Mailman School of Public Health, New York State Psychiatric Institute.