Is there a causal relationship between medical marijuana laws and substance use among youth and young adults?

Medical marijuana (MM) laws are intended to help patients cope with chronic pain and loss of appetite accompanying serious illness. They could, however, have unintended consequences for adolescents and young adults, who are at a formative stage in their decision-making. The majority of users of illicit substances initiate use in adolescence or young adult years, and use during these years is a predictor of future substance misuse problems. This serial cross-sectional study examines the impact of medical marijuana (MM) laws on initiation, consumption, and substance use disorders among U.S. adolescents and young adults, for marijuana and other drugs. Our central hypothesis is that state MM laws will be associated with earlier age at first use of marijuana, greater frequency of use (among users), and greater likelihood of substance use disorders (e.g., abuse, dependence). At the same time, we hypothesize that marijuana will be an economic substitute for alcohol and other illicit drugs, thereby leading to reduced consumption and harms attributable to these other substances.

We examine individual-level data from youth and young adults, ages 13-25, on initiation of drug use, consumption of drugs, and substance use disorders, for marijuana and other substances. These data are obtained from the restricted-use version of the National Survey on Drug Use and Health (NSDUH). Our research fills several gaps in the current literature on this topic: (1) We examine the effects of the characteristics of MM laws, by describing state MM laws along three dimensions – ease of becoming a permitted user, quantity of product permitted, and organization of distribution system; (2) We disaggregate our population into age groups based on the literature on child/adolescent development in order to more precisely estimate the impact of MM laws on relevant populations; (3) We use multiple methods to assess whether any relationship between MM laws and outcomes is causal.