Medical marijuana laws in 50 states: Investigating the relationship between state legalization of medical marijuana and marijuana use, abuse and dependence

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Abstract

Background

Marijuana is the most frequently used illicit substance in the United States. Little is known of the role that macro-level factors, including community norms and laws related to substance use, play in determining marijuana use, abuse and dependence. We tested the relationship between state-level legalization of medical marijuana and marijuana use, abuse, and dependence.

Methods

We used the second wave of the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC), a national survey of adults aged 18+ (n = 34,653). Selected analyses were replicated using the National Survey on Drug Use and Health (NSDUH), a yearly survey of ∼68,000 individuals aged 12+. We measured past-year cannabis use and DSM-IV abuse/dependence.

Results

In NESARC, residents of states with medical marijuana laws had higher odds of marijuana use (OR: 1.92; 95% CI: 1.49–2.47) and marijuana abuse/dependence (OR: 1.81; 95% CI: 1.22–2.67) than residents of states without such laws. Marijuana abuse/dependence was not more prevalent among marijuana users in these states (OR: 1.03; 95% CI: 0.67–1.60), suggesting that the higher risk for marijuana abuse/dependence in these states was accounted for by higher rates of use. In NSDUH, states that legalized medical marijuana also had higher rates of marijuana use.

Conclusions

States that legalized medical marijuana had higher rates of marijuana use. Future research needs to examine whether the association is causal, or is due to an underlying common cause, such as community norms supportive of the legalization of medical marijuana and of marijuana use.

Introduction

Marijuana is the most frequently used illicit substance, and marijuana abuse and dependence are highly prevalent in the United States (American Psychiatric Association, 2000, Compton et al., 2007, Johnston et al., 2009, Johnston et al., 2010, Office of Applied Studies, 2008). Chronic, regular use is associated with DSM-IV diagnoses of marijuana use disorders (Grant and Pickering, 1998). Such disorders are associated with marijuana withdrawal, unemployment, personality dysfunction, crime, respiratory problems and other psychiatric disorders (Budney et al., 2004, Budney and Moore, 2002, Hall and Lynskey, 2009, Haney, 2005, Hasin et al., 2008, Pedersen and Skardhamar, 2010, Taylor et al., 2000).

Behaviors are determined, at least in part, by expectations about the costs and benefits of one's actions, including social approval or disapproval (Akers et al., 1979, Bandura, 1977, Bandura, 1986). However, individual behaviors are also likely to be influenced by group-level acceptance or approval, also known as group norms (Armitage and Conner, 2001). Regarding marijuana, more positive beliefs and greater likelihood of use are more likely among individuals in communities or geographic areas with more approving norms (Lipperman-Kreda and Grube, 2009, Lipperman-Kreda et al., 2010).

Studies of individual perceptions of norms suggest that such norms predict marijuana use (Beyers et al., 2004, Botvin et al., 2001, Elek et al., 2006, Hansen and Graham, 1991). These studies, while important, do not provide information on group-level norms, which is needed for several reasons. First, individual perceptions of societal norms may not always be accurate. Second, societal norms may influence behavior independently of individual beliefs. That is, other things being equal, a given individual may be more likely to use marijuana in an accepting than in a non-accepting society, as we recently showed (Keyes et al., 2011). Third, policy and program interventions focused on societal norms may have a wider impact than interventions focused on individuals (Chilenski et al., 2010, Lipperman-Kreda and Grube, 2009, Lipperman-Kreda et al., 2010). Therefore, studying the influence of societal-level norms is increasingly important, especially during times such as the present when marijuana use, abuse and dependence are increasing. However, a difficulty in conducting this research is the scarcity of informative societal-level data on groups with differing norms.

State medical marijuana laws can be seen as one indicator of group-level approval of marijuana use. These laws legalize marijuana use, when authorized by a physician, for medical purposes such as alleviation of nausea and vomiting from chemotherapy, wasting in AIDS patients, and chronic pain unresponsive to opioids (Procon.org). Between 1996 and 2011, 16 states passed laws legalizing marijuana use for medical purposes. Medical marijuana laws can be used to represent state-level norms on marijuana use because generally, a substantial relationship exists between public opinion and policy decisions (Brooks, 2006, Burstein, 2003, Burstein, 2006, Nielsen, 2010) and specifically, because community norms regarding substance use (e.g., drinking and cigarette smoking) are directly related to policy and enforcement efforts (Lipperman-Kreda and Grube, 2009, Lipperman-Kreda et al., 2010). For example, Khatapoush et al. found that among individuals aged 16–25 in California, marijuana use did not increase in 1996 after legalization of medical marijuana, but marijuana use was higher in California than in other 10 comparison states in 1995, 1997 and 1999 (Khatapoush and Hallfors, 2004). This suggests that state-level norms may have contributed to both the legalization of medical marijuana and to higher rates of use in California in comparison to other states.

We used data from a national, population-based study to examine the relationship between state-level legalization of marijuana, and state- and individual-level population-based rates of marijuana use and marijuana abuse/dependence. We addressed the following questions: (1) did states that legalized medical marijuana by 2004 exhibit higher rates of past-year marijuana use and abuse/dependence in 2004–2005 than states that did not legalize it?; (2) were individuals living in states that legalized medical marijuana at higher risk for marijuana use, abuse and dependence in the past year than individuals who live in states that did not legalize medical marijuana?; and (3) among marijuana users, was residence in a state that legalized medical marijuana associated with increased risk for meeting criteria for marijuana abuse and dependence?

Section snippets

Primary exposure variable: state-level medical marijuana laws

Our primary exposure variable was whether a state had legalized the medical use of marijuana by 2004. This year was chosen to coincide with the period in which our main data source was collected. The following states were defined as “exposed”: Alaska, California, Colorado, Hawaii, Maine, Montana, Nevada, Oregon, Vermont and Washington (Fig. 1). The remaining 40 states were “unexposed” in 2004.

Outcome data

For our outcome variables, we analyzed data from two surveys that used different methods to collect

State-level results

The first two columns of Table 1 present mean state-level prevalence of past-year marijuana use and abuse/dependence, obtained as predicted values from our state-level linear regression models. Using NESARC, the average state-level prevalence of past-year marijuana use differed significantly between states with (7.13%) and without (3.57%) medical marijuana laws (P < 0.0001). The average NESARC state-level prevalence of marijuana abuse/dependence also differed significantly between states with

Discussion

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

Role of funding source

This study was supported by K05 AA014223, R01DA018652, U01AA018111 and the New York State Psychiatric Institute (Hasin) and 1K01DA030449-01 (Cerdá). The NIAAA, NIDA, and the New York State Psychiatric Institute had no further role in study design; in the collection, analysis and interpretation of data; in the writing of the report; or in the decision to submit the paper for publication.

Contributors

Drs. Cerdá and Hasin designed the study. Drs. Cerdá and Keyes managed the literature searches and summaries of previous related work. Dr. Wall undertook the statistical analysis, and Dr. Cerdá wrote the first draft of the manuscript, with sections also written by Drs. Wall and Keyes. Drs. Hasin and Galea provided critical input to multiple drafts of the paper. All authors contributed to and approved the final manuscript.

Conflict of interest statement

All authors declare that they have no conflicts of interest.

Acknowledgements

We would like to thank Eliana Greenstein for her assistance in the data analysis. We also thank NIAAA, NIDA and the New York State Psychiatric Institute for funding this study.

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