Legal medical marijuana does not drive down opioid overdose deaths, according to a new study in the Proceedings of the National Academy of Sciences, refuting an earlier study that found evidence they did.
The earlier research, published in 2014, was based on the fact that between 1999 and 2010, ten states legalized medical marijuana; a further three states had previously had medical regimes. By taking advantage of the implementation of medical marijuana in different states at different times, the authors examined how the introduction of such a law seemed to affect the opioid overdose death rate.
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The results were startling: States with medical marijuana had a 24.8% lower mean annual opioid overdose mortality than states without. This relationship persisted even when controlling for other factors including unemployment, and whether or not the state implemented opioid abuse reduction tools like Prescription Drug Monitoring Programs.
As the opioid crisis worsened, this finding became a key component of arguments from some advocates that medical and/or recreational marijuana legalization would be necessary to cut overdose deaths. Proponents argued that if marijuana were available to pain patients, they would use it as an analgesic instead of opioids—and all the better if it were freely available without a prescription. So strong is this posited connection that last year, New York State added opioid use to its list of qualifying conditions for a medical marijuana card.
The 2014 paper, however, had a key limitation: It only looked at the relationship for 11 years of data, from 1999 to 2010. Today, we have an additional 7 years to look at. A team at Stanford decided to rerun the same analysis, but using the larger data set. They found that in doing so, the relationship between medical laws and opioid overdose rates actually reversed: Using the data through 2017 meant that medical marijuana laws were associated with a 23 percent higher rate of opioid overdose.
"Had the analysis' endpoint been between 2008 and 2012, the results would have been comparable to those obtained by Bachhuber et al. [the original paper]. However, the association became equivocal in 2013; by 2017 it had reversed such that a study conducted in that year might lead some to conclude that medical cannabis laws were compounding opioid overdose mortality," the study's authors write.
This finding persists even in controlling for different types of medical marijuana laws (e.g. whether or not a state has both legal and medical, or whether medical marijuana is required to be low-THC).
Just like its 2014 counterpart, the study looks at rates of death from "opioid analgesics" which, lead author Dr. Chelsea Shover told the Free Beacon, covers the overdose death categories that include prescription opioids, methadone, and synthetic opioids. This last category may complicate analysis because it contains fentanyl and its analogs, which caused barely any deaths in 2010 but were involved in nearly 30,000 as of 2017.
It's not clear if this change in the composition of "opioid analgesics" should have implications for the analysis. Dr. Shover said that the study's model compensated for the "worsening overdose epidemic over time" using a technique called year fixed effects.
Still, in interpreting the results as presented, one has to ask: Can it really be that medical marijuana used to reduce opioid overdoses, and now actually increases them? Maybe, but probably not. A more likely explanation, the authors argue in their discussion, is that the relationship was wholly spurious to begin with. They note that only about 2.5 percent of the population actually uses medical marijuana—a small fraction, unlikely to have an effect on a population-wide phenomenon like opioid overdose rates.
Instead, the study's authors think that some other, unmeasured factors—they mention "state incarceration rates and practices, naloxone availability, and the extent of insurance and services"—are probably at play.
"If you think opening a bunch of dispensaries is going to reduce opioid deaths, you'll be disappointed," co-author Dr. Keith Humphreys said. "We don't think cannabis is killing people, but we don't think it's saving people."
The major impact of the study, then, is to challenge the idea that marijuana legalization is a useful policy intervention vis-à-vis curbing the overdose crisis.
"There are valid reasons to pursue medical cannabis policies, but this doesn't seem to be one of them," Dr. Shover said. "I urge researchers and policymakers to focus on other ways to reduce mortality due to opioid overdoses."