This study indicates that states that legalized marijuana use for medical purposes have significantly higher rates of marijuana use and of marijuana abuse and dependence. The results for marijuana use were found at the state level in two national datasets, the NESARC and the NSDUH, and at the individual level in the NESARC. In addition, in the NESARC, respondents living in states with medical marijuana laws had significantly higher prevalence of marijuana use disorders (abuse/dependence) as defined by DSM-IV. However, in the NESARC, among those who were using marijuana, there was no increase in odds of abuse or dependence, suggesting that any relationship detected between state medical marijuana laws and marijuana abuse/dependence is explained by differences in marijuana use.
Our findings do not necessarily indicate a causal effect of legalization of medical marijuana on marijuana use or marijuana abuse/dependence; that would require a different study design. However, the findings do raise the need to consider possible explanations or mechanisms for the relationships we found, all of which could serve as the basis for further studies. We consider four potential mechanisms.
First, state-level community norms more supportive of marijuana use may contribute to the legalization of medical marijuana and to higher rates of marijuana use. Prior studies on drinking and smoking suggest a direct link between community approval of use and policy change (Lipperman-Kreda and Grube, 2009
; Lipperman-Kreda et al., 2010
). Regarding marijuana, passage of state medical marijuana laws may reflect underlying state-level community norms, especially when such legislation is passed by voter referenda. In addition, the medical marijuana laws that passed in state legislatures by wide margins of votes appear to reflect an underlying high level of support for such legislation prior to their enactment, as well as the absence of a strong and vocal minority opposition (Mikos, 2009
; Scott, 2000
Second, the enactment of medical marijuana laws could lead to a change in community attitudes on both medical and non-medical marijuana use, including reduced disapproval and perceived riskiness of use, which could subsequently influence marijuana use and abuse/dependence. Prior work has shown a relationship between formal behavioral sanctions and the subsequent creation of informal social norms and regulation of behavior (Scott, 2000
). However, the scarce existing evidence on the link between marijuana laws, attitudes and marijuana use raises questions about the validity of this type of causal link. Khatapoush et al., for example, found that while perceived harm of marijuana decreased after legalization of medical marijuana in California, approval of recreational use and actual recreational use did not change with the change in the laws (Khatapoush and Hallfors, 2004
The position that community norms supportive of marijuana use may be an underlying mechanism explaining the higher rates of marijuana use and abuse/dependence in states that legalized medical marijuana (either as a “common cause” of changes in legislation and marijuana use or as a mediator linking legislation to marijuana use) is supported by the broader literature on group norms, which has demonstrated that group norms shape individual behavior and mental health (Asch, 1951
; Cullen, 1983
; Durkheim, 1938
) and that social pressures to conform to the group norms influence the decision to engage in behaviors once norms are internalized. In the area of substance use, parallels can be established with alcohol use and cigarette smoking. For example, “cultures” of drinking (Skog, 1985
) in the neighborhood (Ahern et al., 2008
) and workplace (Barrientos-Gutierrez et al., 2007
) have been linked with risk for binge drinking. Further, perceived disapproval of adolescent alcohol use is associated with less prevalent underage drinking (Kumar et al., 2002
; Lipperman-Kreda et al., 2010
). Permissive neighborhood smoking norms have also been associated with increased prevalence of smoking (Ahern et al., 2009
A third potential mechanism underlying the association between medical marijuana laws and marijuana use and abuse/dependence is medical endorsement of its use for medical purposes. However, no consensus exists at this time on the effectiveness of marijuana as a treatment for symptoms of pain, nausea, vomiting and other problems caused by illnesses or treatment (Joy et al., 1999
; MacCoun and Reuter, 2001a
; Martín-Sánchez et al., 2009
). The lack of medical consensus means that both pro and con proponents of medical marijuana laws can find research support for their positions, and the medical community has not delivered a clear message to the public.
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
; 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
State legalization of medical marijuana may also be associated with potential health, economic, and social gains that we do not consider in this paper. Benefits of legalization may include relief of pain and nausea for cancer and HIV/AIDS patients, tax revenue from marijuana sales, control of crime, decreased costs of the criminal justice system, and reduction in the disproportionate incarceration of minorities for possession of small quantities of marijuana (Levine and Reinarman, 1991
; van den Brink, 2008
; Wodak, 2002
). While we recognize such potential benefits, our study cannot make any statements on such aspects of legalization of medical marijuana.
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 regarding state of residence at Wave 1. While movement between waves could have led to misclassification of a subset of respondents, related research finding a significant relationship between minimum drinking age laws based on the state of birth and current substance use disorders indicates that misclassification may not be differential by state legalization status (Norberg et al., 2009
). Fourth, we examined the relationship between state legalization by 2004 and marijuana abuse/dependence, and thus excluded four states that have legalized medical marijuana since 2004. To determine whether this affected our results, we re-did the NSDUH analysis using 2007-2008 data, adding the 2 states (Rhode Island and New Mexico) that legalized medical marijuana between 2004 and 2007. The results were not significantly different from those produced from the 2004-2005 data (P
>0.05) and are available upon request. Finally, we compared states by legalization status. Future studies need to assess the variation in marijuana use related to heterogeneity of the laws across states, since allowances vary between states in factors such as home production permits, role of the doctor in determining access to marijuana, and the types of illnesses and conditions for which it is legal to access marijuana.
This study highlights the key role that macro-level factors, such as policy changes and community norms about substance use, play in shaping marijuana use and abuse/dependence. Future studies are also needed on the consequences of increased marijuana use, such as accidents, aggression, school drop out, psychosis, HIV and sexually transmitted disease rates (Fergusson et al., 2003a
; Fergusson et al., 2003b
; Hall and Degenhardt, 2009
) as well as on the particular impact of medical marijuana legalization on youth, who bear a disproportionate burden of marijuana-related disorders (Budney and Moore, 2002
), and are vulnerable to the advertising effects of other substances such as tobacco (Hanewinkel et al., 2011
). In particular, future studies in the United States and elsewhere can build on our findings by comparing trends in community norms, marijuana use and abuse/dependence before and after the legalization of marijuana, to understand the relative contribution of medical marijuana legalization and community norms on changes in marijuana use and abuse/dependence.