A widely cited 2014 study, reported in JAMA Internal Medicine, found that medical marijuana laws were associated with lower rates of opioid-related mortality between 1999 and 2010. Supporters of marijuana reform have frequently invoked that study, written by University of Pennsylvania researcher Marcus Bachhuber and two co-authors, as evidence that letting people use cannabis for medical and/or recreational purposes helps reduce opioid-related deaths. Not so fast, say the authors of a study published yesterday in the Proceedings of the National Academy of Sciences, who extended the analysis for another seven years and found no such association.
Stanford researcher Chelsea Shover and three collaborators replicated the results of Bacchuber et al.'s study for the period from 1999 through 2012. But they found that "the association became equivocal in 2013," and "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."
Shover et al. caution against drawing that conclusion, however. "The observed association between these two phenomena is likely spurious rather than a reflection of medical cannabis saving lives 10 [years] ago and killing people today," they write. "Medical cannabis users are about 2.5% of the population, making it unlikely that they can significantly alter population-wide indices. Unmeasured variables likely explain both associations (e.g., state incarceration rates and practices, naloxone availability, and the extent of insurance and services)."
When Shover et al. limited their analysis to states that only allow medical use of low-THC cannabis extracts, they found a negative correlation with opioid-related deaths. Looking just at states with "comprehensive medical cannabis law[s]," they found a positive correlation. In states that have legalized cannabis for recreational as well as medical use, there was a negative correlation. Only the second result was statistically significant, and barely so. Nevertheless, this is not the pattern you would expect to see if increased legal access to marijuana had a measurable impact on deaths involving opioids, either negative or positive.
Shover et al.'s method differs in a potentially important way from the approach taken in two other studies suggesting that access to medical marijuana reduces opioid-related mortality. While Shover et al., like Bacchuber et al., focus on states with medical marijuana laws, the two other studies asked whether patients actually had ready access to cannabis.
A 2018 study, reported in the Journal of Health Economics, found that merely having a medical marijuana law was associated with lower rates of opioid-related death until 2010. After that there was no apparent benefit from medical marijuana laws per se, but states with "legally protected and operational dispensaries" continued to see reductions, suggesting that "broader access to medical marijuana facilitates substitution of marijuana for powerful and addictive opioids." That study was based on data through 2013, so Shover et al.'s analysis includes four more years.
A 2019 study, reported in the Economics Bulletin, likewise found that "states with active legal dispensaries see a drop in opioid death rates over time." That study covered 1999 through 2015, ending two years before Shover et al.'s analysis does.
In addition to covering more years, Shover et al. define medical marijuana access more broadly than those two earlier studies did. While Shover et al. distinguish between states with "low-THC-only medical cannabis law[s]" (where CBD oil is legal, sometimes only notionally, for a short list of conditions) and states with "comprehensive medical cannabis law[s]" (where a broader range of cannabis products are legal for a broader range of conditions), they implicitly treat passage of such laws as equivalent to legal access, which is often misleading.
Shover et al.'s data set, for example, indicates that they counted Arizona, where the first medical marijuana dispensary did not open until December 2012, as a state with a "comprehensive medical cannabis law" from 2011 through 2017. Arkansas, where the first dispensary opened last month, gets credit for such a law in 2017 and part of 2016. Hawaii is listed as a state with a comprehensive law from 2001 on, but legal sales did not begin there until 16 years later. There are similar issues with Shover et al.'s treatment of other states, including Connecticut, Delaware, Florida, Illinois, and Massachusetts.
In short, Shover et al. classify states based on their enactment of medical marijuana laws, even though legal sales may not begin until years later. That makes sense insofar as they are seeking to replicate the results of Bachhuber et al.'s study, which took the same approach. But it could easily muddy the picture of what happens when medical marijuana is legally available.
On the face of it, a negative correlation between medical marijuana and opioid-related deaths is consistent with research finding that medical marijuana is associated with reductions in opioid prescriptions. But since the risk of a fatal overdose among patients taking opioids for pain relief is very low, that explanation seems inadequate to account for such a correlation, assuming it exists. Is marijuana an important substitute for opioids among nonmedical users? Maybe, but on the whole Shover et al.'s agnosticism seems appropriate, even if their method is not the best way to measure the availability of medical marijuana.