A fourth potential mechanism relates to marijuana availability: legalization of medical marijuana may lead to greater commercial promotion and availability of the substance for recreational purposes, which may contribute to greater illicit use of marijuana. Pacula et al. examined state temporal variation in the adoption of active medical marijuana policies, and found that policies aimed at users (e.g., provisions for physicians to recommend marijuana or allowances for a medical necessity defense for those who use marijuana for medical purposes) led to changes in prices of marijuana in local markets, in a fashion that was consistent with anticipated increases in demand (Pacula et al., 2010). Yet related research indicates that decriminalizing marijuana in other countries (and thus increasing its commercial availability) did not lead to increased use (MacCoun and Reuter, 1997, 2001b; McGeorge and Aitken, 1997; Simons-Morton et al., 2010; Single, 1989), although one study in the Netherlands suggested that shifting from depenalization to active commercialization of marijuana was associated with increased marijuana use (MacCoun and Reuter, 2001b).
