Association between medical cannabis laws and opioid overdose mortality has reversed over time, Chelsea L. Shover et al., 2019

Association between medical cannabis laws and opioid overdose mortality has reversed over time

Chelsea L. Shover, Corey S. Davis, Sanford C. Gordon, and Keith Humphreys

PNAS, mai 2019



Medical cannabis has been touted as a solution to the US opioid overdose crisis since Bachhuber et al. [M. A. Bachhuber, B. Saloner, C. O. Cunningham, C. L. Barry, JAMA Intern. Med. 174, 1668–1673] found that from 1999 to 2010 states with medical cannabis laws experienced slower increases in opioid analgesic overdose mortality. That research received substantial attention in the scientific literature and popular press and served as a talking point for the cannabis industry and its advocates, despite caveats from the authors and others to exercise caution when using ecological correlations to draw causal, individual-level conclusions. In this study, we used the same methods to extend Bachhuber et al.’s analysis through 2017. Not only did findings from the original analysis not hold over the longer period, but the association between state medical cannabis laws and opioid overdose mortality reversed direction from −21% to +23% and remained positive after accounting for recreational cannabis laws. We also uncovered no evidence that either broader (recreational) or more restrictive (low tetrahydro-cannabinol) cannabis laws were associated with changes in opioid overdose mortality. We find it unlikely that medical cannabis—used by about 2.5% of the US population—has exerted large conflicting effects on opioid overdose mortality. A more plausible interpretation is that this association is spurious. Moreover, if such relationships do exist, they cannot be rigorously discerned with aggregate data. Research into therapeutic potential of cannabis should continue, but the claim that enacting medical cannabis laws will reduce opioid overdose death should be met with skepticism.

Keywords : medical cannabis, opioid overdose, public policy


A 2014 study by Bachhuber et al. (1) created a sensation by showing that state medical cannabis laws were associated with lower-than-expected opioid overdose mortality rates from 1999 to 2010. Cited by more than 350 scientific articles to date, the study attracted national and international media attention and was hailed by many activists and industry representatives as proof that expanding medical cannabis would reverse the opioid epidemic (1). Despite the authors’ cautions about drawing firm conclusions from ecological correlations, and similar warnings from other scientists (2), many such conclusions were drawn, to the point where medical cannabis has now been approved by several states as a treatment for opioid use disorder (3, 4). Subsequently published papers also found that cannabis access and indices of opioid-related harm were negatively correlated in the aggregate, with some attributing this less to medical cannabis laws per se than to increased access to any form of cannabis (including recreational) (5–7).

Given mounting deaths from opioid overdose, replicating the Bachhuber et al. (1) finding is a worthy task, especially in light of the changing policy landscape. Between 2010 and 2017, 32 states enacted medical cannabis laws, including 17 that allowed only medical cannabis with low levels of the psychoactive tetrahydrocannabinol (THC) and high levels of the nonpsychoactive component cannabidiol. Eight states enacted recreational cannabis laws during this period. Opioid overdose deaths have also increased dramatically over that time period (8). Using the same methods as Bachhuber et al. (1), we revisited the question with seven more years of data. To investigate how newer cannabis laws may be associated with changes in the association between cannabis laws and opioid overdose mortality, we also created a model with additional terms to account for presence of a recreational cannabis law or a low-THC restriction. Because none of the states with low-THC laws operate medical dispensaries and many limit access to a small number of indications, the levels of access can be approximated as highest for recreational, then “comprehensive” medical with dispensaries, and lowest for states with low-THC only. If broader access to cannabis writ large, rather than medical cannabis specifically, is the latent factor associated with lower opioid overdose mortality, we would expect to see the most negative association in states with recreational laws and the least negative association (or even positive) association in states with low-THC-only laws.