Evidence before this study

If passage of a state law legalising marijuana for medical use conveyed a public message to adolescents that marijuana use was acceptable or did not lead to adverse effects, this law could quickly increase adolescent use of marijuana, even if the law was implemented slowly or had provisions that tightly restricted marijuana use. To identify studies relevant to this issue, we searched PubMed for English-language articles with the term, “medical marijuana”. As of April 6, 2015, 449 articles with this term were published, the first in 1978, and all the rest since 1994. Most articles were opinion pieces about the pros and cons of medical marijuana use, regarding either its medicinal benefits, or implications for society. To be considered relevant to the present study, we reviewed reports with empirical findings that were based on general-population surveys with state-based samples, had marijuana use as an outcome, and compared states with and without medical marijuana laws, or states before and after passage of such laws. We identified two reports showing overall higher rates of marijuana use in states with medical marijuana laws, one in adolescents and the other in adults, with one replication of the adolescent result. Comparison of Colorado, a state with a medical marijuana law, with states without medical marijuana laws found suggestive but inconclusive evidence regarding the effects of the law on adolescent marijuana use. Two studies, that in combination examined seven states that passed medical marijuana laws did not show increased prevalence of adolescent marijuana use after the laws were passed, relative to the prevalence before the laws were passed. However, limitations in the number of states examined, number of years, and sample sizes left unclear whether the absence of differences in marijuana use pre-law and post-law were real or due to limitations of the methods.

Added value of this study

Our study, which included data from annual national surveys spanning 24 years (1991–2014) for 1 098 270 adolescents in 48 US states, provides two pieces of definitive evidence about medical marijuana laws and adolescent use of marijuana. First, across all survey years, overall adolescent marijuana use was higher in states that had ever passed medical marijuana laws than in states that did not have these laws, but the increased use was present in states both before and after the laws were passed. Second, our comprehensive study showed no evidence for an increase in adolescent use of marijuana in the year of passage of a medical marijuana law, or in the first or second years after passage. These results were consistent across several sensitivity analyses that used a different definition of the marijuana outcome variable, that removed one state at a time from the sample to establish whether one state was unduly affecting the overall results (none did), or whether a state medical marijuana law provided for dispensaries.

Implications of all the available evidence

Our two main findings, in conjunction with other evidence, suggest that state-level factors other than medical marijuana laws influence adolescent marijuana use. Because both human studies and animal models show that early adolescent use of marijuana increases the risk of important adverse effects in adulthood, the identification of large-scale societal factors that increase the risk of early use is crucial. Our study findings suggest that the debate over the role of medical marijuana laws in adolescent marijuana use should cease, and that resources should be applied to identifying the factors that do affect risk.