Between 2002 and 2008, adolescent marijuana use was higher and perception of its riskiness lower in states with medical marijuana laws compared to states without such laws. There are several possible explanations for this observation. First, it is possible that states with higher marijuana use and lower perceptions of risk are more likely to enact MML. This explanation is supported in the current analysis by the observation that among states that eventually enacted MML, use was higher and perceptions of risk lower even prior to passage of MML. Second, it is possible that MML causes changes in marijuana use and perceptions thereof. A longer time window of pre/post data would be needed to provide enough information both before and after passage of MML for each state to investigate this possibility. Third, it is possible that common factors drive both use and perceptions of risk and implementation of MML. For example, changing cultural norms around drug use may influence both. Given the fast-changing nature of medical marijuana laws and the fact that most claims about its association with adolescent marijuana use
