摘要：Objectives. Medical marijuana laws (MMLs) have been suggested as a possible cause of increases in marijuana use among adolescents in the United States. We evaluated the effects of MMLs on adolescent marijuana use from 2003 through 2011. Methods. We used data from the Youth Risk Behavior Survey and a difference-in-differences design to evaluate the effects of passage of state MMLs on adolescent marijuana use. The states examined (Montana, Rhode Island, Michigan, and Delaware) had passed MMLs at different times over a period of 8 years, ensuring that contemporaneous history was not a design confound. Results. In 40 planned comparisons of adolescents exposed and not exposed to MMLs across states and over time, only 2 significant effects were found, an outcome expected according to chance alone. Further examination of the (nonsignificant) estimates revealed no discernible pattern suggesting an effect on either self-reported prevalence or frequency of marijuana use. Conclusions. Our results suggest that, in the states assessed here, MMLs have not measurably affected adolescent marijuana use in the first few years after their enactment. Longer-term results, after MMLs are more fully implemented, might be different. The presence of marijuana in the United States was first recorded in 1611 and the substance remained largely unregulated until the beginning of the 20th century (for a review of historical legislation on marijuana in the United States, see Pacula et al. 1 ). Before the 20th century, marijuana was commonly prescribed by physicians for a variety of medical conditions. A rise in nonmedical marijuana use in the early part of the 20th century, coupled with societal reform on the recreational use of many substances (including alcohol), resulted in the first legislation aimed at regulating marijuana in the United States. Since then the legal status of marijuana has evolved; the drug became progressively more regulated through the 1970s, at which point marijuana was categorized as a Schedule I drug (illegal under federal law, with no currently accepted medical use) according to the Federal Controlled Substances Act. 2 After the federal government classified marijuana as a Schedule I drug, states began passing their own legislation that addressed the medical use of marijuana. 1 Most state legislation was specific to research programs or did not include provisions protecting physicians and patients from penalties, and as such these laws did not really promote the medical use of marijuana. California passed the first medical marijuana law (MML) that included broad protections for patients and physicians in 1996, and since then 17 states and the District of Columbia have passed MMLs. MMLs typically contain legal provisions that cover 4 dimensions of medical marijuana regulation ( Table 1 ). The first is the establishment of a state registry or identification cards for patients prescribed medical marijuana. The second dimension refers to state regulation of the practice of medicine (e.g., regulation of the patient–physician relationship, disease or condition limitations, and prohibition of disciplinary action or denial of privileges of a physician). Third, states can be arrayed according to their regulation of access to marijuana, including possession and cultivation limitations as well as retail–dispensary regulation. Finally, state MMLs are distinguished on the basis of affirmative defenses and legal protections for the medical marijuana patient, including protection from prosecution. TABLE 1— Medical Marijuana Laws Enacted in the United States, November 1996 Through May 2011 Type of Provision State Legislation Effective Date Registry/Identification Cards Regulation of Practice of Medicine Possession and Cultivation Limits Dispensary/Retail Affirmative Defenses/Legal Protections California California Compassionate Use Act of 1996, Cal. Health & Safety Code, §11362.5 (1996), Cal. Health & Safety Code, §§11362.7–11362.83 November 6, 1996 (amended January 1, 2004) Yes Yes Yes Yes Yes Washington Wash. Rev Code §§69.51A– 69.51A.901 (2007) November 3, 1998 (amended November 2, 2008, and June 10, 2010) No Yes Yes No Yes Oregon Ore. Rev Stat §475.300 (2007) December 3, 1998 (amended 1999) Yes Yes Yes No Yes Alaska Alaska Stat §§17.37.10–17.37.80 (2007) March 4, 1999 Yes Yes Yes No Yes Maine Me. Rev Stat Title 22 Health and Welfare, Ch. 558-C, Maine Medical Use of Marijuana Act December 22, 1999 Yes Yes Yes Yes Yes Hawaii Haw. Rev Stat §§329-121–329-128 (2008) December 28, 2000 Yes Yes Yes No Yes Colorado Colo. Const. art. XVIII, §14 (2001), Colo. Rev Stat §18-18-406.3 (2001), Colo. Rev Stat §25-1.5-106 (2003), Colo. Rev Stat §12-43.3 (2010) June 1, 2001 Yes Yes Yes Yes Yes Nevada Medical Use of Marijuana, ch. 453A, Nev. Rev Stat Ann. §§453A.010–810 (2011) October 1, 2001 Yes Yes Yes No Yes Maryland Md. Code Ann., Criminal Law §5-601(c) 3 (II) (2003) January 1, 2003 No No No No Yes Vermont Vt. Stat Ann. Title 18, §§4471–4474d (2003) July 1, 2004 Yes Yes Yes Yes Yes Montana Montana Medical Marijuana Act, Mont. Code Ann. §§50-46-101–210 (2010) November 2, 2004 Yes Yes Yes Yes Yes Rhode Island Edward O. Hawkins and Thomas C. Slater Medical Marijuana Act, R.I. Gen. Laws Ann. §§21-28.6-1–21-28.6-12 (2006) January 3, 2006 Yes Yes Yes Yes Yes New Mexico N.M. Stat Ann. §30-31C-1 (2007) July 1, 2007 Yes Yes Yes Yes Yes Michigan Mich. Comp. Law §§333.26421–333.26430 (2008) December 4, 2008 Yes Yes Yes No Yes District of Columbia District of Columbia Official Code, Title 7, Human Health Care and Safety, Subtitle G-ii, Chapter 16B, Use of Marijuana for Medical Treatment, D.C. Code §§7-1671.02–7-1671.13 (2010) May 1, 2010 Yes Yes Yes Yes Yes New Jersey N.J. Stat Ann. §24:6I (2010) October 1, 2010 Yes Yes Yes Yes Yes Arizona Ariz. Rev Stat §§36-2801–2819 November 2, 2010 Yes Yes Yes Yes Yes Delaware Delaware Medical Marijuana Act, SB 17 (not yet codified) May 13, 2011 Yes Yes Yes Yes Yes Open in a separate window Concurrent with the gradual increase in state MMLs since 1996, national trends indicate declines in nonmedical marijuana use among adolescents from 1996 to 2008 followed by increases from 2008 to 2010. 3 Some have suggested that passage of state MMLs may be contributing to recent increases in nonmedical marijuana use among adolescents, sparking a national debate regarding the legal status of marijuana. Moreover, noting the lag between the enactment of California’s MML in 1996 and the recent increases in adolescent marijuana use beginning in 2008, there may be a delayed effect of MMLs on adolescent marijuana use. Theoretically, MMLs might make marijuana cheaper and more easily accessible to adolescents. The laws might reduce perceptions of the harmfulness or toxicity of marijuana and might increase the social acceptability of marijuana use. On the basis of such suppositions, MMLs might be expected to increase nonmedical marijuana use among adolescents. By contrast, the laws’ function of labeling marijuana as a medication for severely ill patients could reduce the perception of marijuana as a recreational drug, thus resulting in reduced nonmedical marijuana use among adolescents. Moreover, MMLs might not increase adolescents’ ease of access to marijuana if, as a result of minimal enforcement of marijuana prohibitions, the drug is already functionally decriminalized. Early research on the potential impact of MML passage on marijuana use did not reveal any effects. 4,5 A report from the Marijuana Policy Project Foundation evaluated marijuana use via multiple state and national surveys in 12 states that had passed MMLs subsequent to 1996 and showed either no change or statistically significant decreases in marijuana use after enactment of policies. 6 In a recent study, Wall et al. used data from the National Survey on Drug Use and Health to compare the state-level prevalence of adolescent marijuana use from 2002 to 2008 in states with and without MMLs. 7 Rates of marijuana use were significantly higher in states that enacted MMLs both before and after the policies had been enacted. The Wall et al. results highlighted the fact that states that eventually pass MMLs may differ in fundamental ways from those that do not pass such legislation. This preexisting cultural difference, rather than a causal link between MMLs and marijuana use, could account for higher rates of marijuana use. To test this hypothesis, Harper et al. 8 conducted further analyses of the same data used by Wall et al., 7 employing a difference-in-differences study design and aggregating state marijuana use rates by age (12–17 years, 18–25 years, ≥ 26 years). Results showed no evidence of a significant increase in marijuana use immediately after the enactment of MMLs in any of the 3 age groups. Despite being a topic of continuing policy debate, the empirical literature on MML effects remains quite limited. We used a survey data set different from that used by Wall et al. and Harper et al. and analyzed data at the individual survey respondent level rather than aggregate reported statewide prevalence rates, providing additional statistical power to detect what might be quite small but important effects of MMLs on adolescent nonmedical marijuana use.
