Legalization of medical marijuana has both fewer risks and fewer benefits than previously reported, new research shows.
A meta-analysis of 11 studies conducted between 1991 and 2014 shows there is no evidence to support the hypothesis that US medical marijuana laws (MMLs) have led to an increase in the prevalence of marijuana use among adolescents.
“For several years, people have been very concerned about the potential for medical marijuana laws to lead to increases in teen use of marijuana, because early heavy use is associated with impaired neurodevelopment, educational, and occupational achievement,” senior author Deborah Hasin, PhD, professor of epidemiology in psychiatry, Columbia University, New York City, told Medscape Medical News.
“The results were quite clear — MML did not lead to increases in teen marijuana use,” she said.
In an accompanying editorial, investigators concluded that it is “premature” to believe that increased access to medical marijuana may help solve the US opioid crisis.
“The purpose of our editorial was to point out how weak such correlational evidence is in supporting the claim that medical cannabis has reduced opioid overdose deaths,” lead author Wayne Hall, PhD, professor, Center for Youth Substance Abuse Research, University of Queensland, Australia, told Medscape Medical News.
He cautioned US state legislative bodies against “drawing the policy conclusion that they could reduce opioid overdose deaths by either allowing medical use of cannabis or by expanding access to medical cannabis.”
The study was published online February 22 in Addiction.
Unanswered Questions
As of November 2016, 28 states had enacted policies legalizing marijuana use for medical purposes, but “public debate has focused on the potential for MMLs to increase recreational use of marijuana in adolescents,” Hasin and coauthors write.
Hasin’s group previously conducted a study of the effects of MMLs in more than 1 million US teens, who were surveyed from 1991 to 2014. The investigators and found that MMLs did not increase teen marijuana use.
Nevertheless, “questions about MML effects remained, and we wanted a more definitive answer, so we did a thorough literature search, found 11 other studies that also examined nationally representative samples of US teens, and conducted a meta-analysis of the results from these studies,” she reported.
All studies were derived from four ongoing large national surveys: Monitoring the Future (MTF) (n = 2); the National Longitudinal Survey of Youth (NLSY) (n = 2); the National Survey on Drug Use and Health (NSDUH) (n = 4); and the Youth Risk Behavior Survey (YRBS) (n = 3).
Four studies excluded years prior to 2002; thus, these studies excluded data collected prior to the earliest passing of MMLs, in California, Oregon, Washington, and Colorado. In addition, two YRBS studies did not include these states.
All the studies used a difference-in-difference (DID) approach in estimating the effect of the passage of MMLs. In that approach, differences in marijuana use from pre- to post-MML passage were compared to contemporaneous differences in non-MML states.
Estimates were standardized and were entered into a meta-analysis model with fixed effects for each study.
None of the studies found significant changes in the estimates of the prevalence of use among adolescents before and after passage of MMLs, as compared with contemporaneous changes in non-MML states (P < .05). The studies did show, however, a nonsignificant pooled estimate (standardized mean difference) of -0.003 (95% confidence interval [CI] = −.012 to +.007).
The particular surveys (ie, MTF, NLSY, NSDUH, or YRBS) were not found to significantly affect study effect size (P = .46).
Four studies compared MML states with non-MML states with regard to differences in the prevalance of past-month marijuana use before the passage of MMLs. All four studies found that rates of past-month marijuana use were higher.
Caution Required
Although the study “shows a lack of medical marijuana laws’ effect on teens’ use,” the findings “do not tell us about what the effects of recreational marijuana laws will be,” she warned.
The findings also “do not diminish the importance of addressing early, heavy use of marijuana in teens, wherever it occurs.”
Additionally, “the few studies available suggest that medical marijuana laws do increase cannabis use and adverse cannabis outcomes in adults,” she said.
Commenting on the study for Medscape Medical News, Stephanie Hartselle, MD, clinical assistant professor of psychiatry, Brown University, Providence, Rhode Island, who was not involved with the study, expressed “serious” concerns regarding its conclusions.
The exclusion from some of the studies of four states in which medical marijuana use has been legal longest “may have slanted the study in a dramatic way, but it’s difficult to assess, given how long these laws have been in place,” she said.
In addition, ” ‘past-month use’ is a very short parameter to use, given the time it takes for laws to be passed and for approval of medical marijuana authorizing clinics and venues for purchase to be established,” she observed.
She noted that “many physicians involved in authorizing the use of medical marijuana in most states will do so for adults only and not for those under the age of 18,” so “finding a prescriber to approve a diagnosis qualifying for a medical marijuana card may have been difficult, if not impossible, in these states.”
For that reason, “evaluating teen use in this context over 1 month may be a narrative to the system of health rather than elucidating the effect of these laws on teen cannabis use.”
Also commenting on the study for Medscape Medical News, Austin Miller, MPP, a PhD candidate at Washington State University, in Pullman, who was not involved in the study, noted, “The interplay between MML and marijuana use is complex” and “not all medical marijuana use is the same.
“Different provisions within such laws are likely to have different effects on adolescent marijuana use, so the respective effects of such different provisions are nonconclusive with the current evidence,” he added.
Hasin said that her group plans to conduct “continued monitoring of MML and recreational marijuana laws in teens and more studies in adults.”
No Impact on Opioid Use
The editorial by Hall and colleagues challenges the claim that an increase in access to medical cannabis will reduce opioid overdose deaths, calling the evidence “very weak.”
They challenge a previous “ecological study” that showed a correlation over time between the passage of MMLs and decreasing opioid overdose death rates.
Ecological studies “do not show directly that it is increased medical cannabis use in these states that has reduced opioid use in individuals,” the authors state.
Furthermore, these studies “have limited capacity to control for the effects of important differences” between those states with and those without MMLs. Such differences include increased access to methadone- and buprenorphine-assisted treatments for opioid dependence, which might reduce overdose risks, the authors point out.
Although the low risk for overdose from cannabinoids is “well established,” their analgesic efficacy is “modest,” and epidemiologic studies of large samples of patients with chronic pain “have found that those who use cannabis do not use lower opioid doses than opioid users who do not use cannabis.”
Hall emphasized that there “are much better treatments that have been shown to reduce overdose deaths that are not currently being widely used in the states.” These include medication-assisted treatments with methadone or buprenorphine.
Commenting on the editorial for Medscape Medical News, Mark Olfson, MD, MPH, professor of psychiatry and epidemiology, Columbia University, New York City, who was not involved in writing the editorial, agreed that, “on the basis of current evidence, it is premature to expand access to medical cannabis as a general strategy to curtail the opioid crisis.”
However, “at the same time, there may be a narrow clinical role for medical cannabis to help with acute pain management in some opioid-dependent patients,” he said.
He noted that “prospective research is needed with opioid-dependent pain patients to help identify the clinical context in which medical cannabis provides analgesia and ameliorates opioid craving and consumption.”
Hall added that it “would be better to offer medication-assisted treatment to opioid-dependent people than to imprison them, because overdose deaths increase dramatically after opioid-dependent people are released from prison.”
The meta-analysis was supported by the National Institute on Drug Abuse, the National Institute on Alcohol Abuse and Alcoholism, and the New York State Psychiatric Institute. The authors and editorialists have disclosed no relevant financial relationships.
Addiction. Published online February 12, 2018. Full text, Editorial
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