Do medical marijuana laws influence adolescent non-medical marijuana use?

Changes in state medical marijuana legislation (MML) have been suggested as potential causes of the recent increasing national trend of non-medical marijuana use among adolescents, sparking a national debate regarding the legal status of marijuana. Initial studies of the effect of MML in California did not find evidence of increases in non-medical marijuana use after MML was enacted (Khatapoush & Hallfors, 2004; O'Keefe, Earleywine, & Mirken, 2008). A more recent study found that states which enacted MML also tended to have statistically higher average rates of non-medical marijuana use compared to states without MML (Wall et al., 2011). Given limitations in these initial studies, continuing evaluation of MML is warranted. We employed difference in difference statistical methods to more rigorously evaluate causal effects of changes in MML on adolescent non-medical marijuana use than studies to date. State-specific data on past 30 day and lifetime non-medical marijuana use among adolescents were obtained from the Youth Risk Behavior Survey (YRBS) for four states: Montana, Rhode Island, Michigan, and Delaware. These four states were selected because they all passed MML at different times across a span of 8 years (2004, 2006, 2008, and 2011, respectively). Out of 20 planned comparisons, only 1 significant effect was found, an outcome expected by chance alone. Thus, data thus far do not indicate recent changes in MML influenced non-medical use of marijuana among adolescents in these states.