Over the past two decades, 23 states and the District of Columbia have legalized marijuana for medical use in the U.S. (Anderson et al., 2014; Pacula et al., 2013). These policies were adopted at different times, allowing researchers to analyze the effects of policy changes as a natural experiment: differences in medical marijuana policies between states over time allow investigators to draw inferences about whether policy that could facilitate access to marijuana are causally associated with key public health outcomes (Anderson et al., 2014, 2013; Cerdá et al., 2012; Choo et al., 2014; Gorman and Charles Huber, 2007; Harper et al., 2012; Lynne-Landsman et al., 2013; Pacula et al., 2013; Rylander et al., 2014; Schuermeyer et al., 2014; Wall et al., 2011). In one of the more intriguing examples of such a study, Anderson and colleagues examined the association between legalization of medical marijuana and changes in state suicide rates over the period 1990–2007 (Anderson et al., 2014). Their results suggested that legalization of medical marijuana led to a decrease in suicide rates. Specifically, they reported that legalization was associated
