Marijuana may not be an effective tool for those struggling with opioid addiction, according to a new analysis.
While marijuana advocates have claimed that the drug can help fight the opioid epidemic, an analysis published in the Canadian Medical Association Journal finds the science behind this claim is equivocal at best. Researchers at McMaster University found that the weight of the evidence does not support the claim that marijuana helps people "exit" heroin or prescription opioids.
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The new analysis indicates that the so-called exit hypothesis—that cannabis "can be used to manage withdrawal symptoms and therefore help patients with opioid use disorder to stop using opioids"—does not withstand scientific scrutiny.
Researchers conducted what is called a meta-analysis, a statistical approach that aggregates the results from multiple studies to test their shared hypothesis. In this case, the authors were interested in individuals who were on methadone, a common therapeutic drug that replaces illicit opioids such as heroin. Combing through old studies, they looked for those that asked what effect using marijuana had on methadone patients' proclivity to keep using opioids and stay in treatment.
The results are not heartening. Researchers identified 12 studies relating marijuana use to continued use of illicit opioids, the majority of which found that marijuana neither increased nor decreased likelihood of abuse. After excluding studies with a high likelihood of being biased, they found just two—conducted by the same researcher, and covering just 200 subjects—which indicated that marijuana reduced the probability of opioid abuse.
Interestingly, marijuana's effectiveness in opioid treatment varied based on where the study was conducted: using marijuana made a person less likely to remain in treatment in the United States, but more likely in Israel. (That disparity is more likely than not due to differences in program design or pure chance.)
What all of this means concretely is that the balance of the evidence does not support the claim that marijuana reliably helps people hooked on opioids to "exit" their drug of choice. In fact, the paper goes out of its way to emphasize that what evidence exists is generally of poor quality, "with critical issues of inconsistency and imprecision."
"Further studies are needed," the paper cautions, "to address and examine the notion of cannabis use and its effect on treatment outcome in patients with opioid use disorder."
The findings released Tuesday are far stronger than those customarily used to back the "exit hypothesis." To this point, most advocates of a marijuana-as-treatment approach have pointed to a pair of studies which find that states with medical marijuana laws saw opioid prescriptions drop following legalization. The causal connection of those claims may be spurious—rates of opioid prescribing have fallen nationwide over the past decade, for reasons more related to a deliberate reduction in supply than in demand. In fact, a more recent study extended the data beyond 2014 and found that the supposed relationship vanished.
The relative weakness of this evidence—and the apparent weakness of the literature more broadly—has not stopped politicians from acting as though marijuana has the power to fix the opioid crisis. In September, New York state added opioid use disorders to the list of conditions that qualified one for a medical marijuana card; New Jersey followed suit in January.
The sorry state of science on the "exit hypothesis" in many ways mirrors the debate around legal marijuana as a whole. Surgeon General Jerome Adams emphasized during recent congressional testimony that widespread legalization—and arguments in favor of it—has happened in the absence of robust scientific evidence on the long-run effects of marijuana on Americans. The rush to legalize constitutes "a massive public health experiment on our citizenry," Adams said.