We note study limitations. First, we relied on cross-sectional data, and thus we cannot demonstrate a causal relationship between enactment of state medical marijuana laws and individual risk for illicit marijuana use. Future studies should use large-sample survey data collected in years prior to and after enactment of marijuana laws in states with and without such laws, to compare prevalences and trends. The fact that only two states changed their medical marijuana laws between the two NESARC study waves made this type of design difficult to implement with NESARC data. Second, the NESARC reported lower rates of marijuana use than the NSDUH, possibly due to NESARC use of interviewer- rather than self-administered questions on marijuana use. However, this concern is offset by two factors: a) the NESARC measure of marijuana abuse/dependence is highly sensitive among users compared to other measures, including the NSDUH (Grucza et al., 2007) and b) most importantly for the present purpose, we found the same relationship between medical marijuana laws and marijuana use in both the NESARC and the NSDUH. Third, the NESARC only released information
