Available online 6 January 2023, 102728

Author links open overlay panelAbstract

Many states have legalized marijuana over the last two decades, initially for medical purposes and more recently for recreational consumption. Despite prior research, it remains unclear how these policies are related to rates of opioid-involved overdose deaths, which have trended rapidly upwards over time. We examine this question in two ways. First, we replicate and extend previous investigations to show that the prior empirical results are frequently fragile to the choice of specifications and time periods, and probably provide an overly optimistic assessment of the effects of marijuana legalization on opioid deaths. Second, we present new estimates suggesting that legal medical marijuana, particularly when available through retail dispensaries, is associated with higher opioid mortality. The results for recreational marijuana, while less reliable, also indicate that retail sales may be correlated with greater death rates relative to the counterfactual of no legal cannabis. A likely mechanism for these effects is the emergence of illicit fentanyl, which has increased the riskiness of even small positive effects of cannabis legalization on the consumption of opioids.

Section snippetsEstimated Legalization Effects from Prior Research are Ambiguous

It is theoretically ambiguous whether cannabis legalization will raise or lower deaths involving opioids. If marijuana and opioids are substitutes, legalization of the former will reduce consumption of the latter, while the reverse will be true if they are complements. However, even if legal marijuana decreases overall opioid use, fatalities involving opioids could rise. For instance, this might occur if opioid consumption falls among those at low risk of death while it rises for those with

Methods

Following previous research, marijuana legalization effects are initially estimated from difference-in-differences (DiD) models of the form:Dit=α+Mitβ+Xitγ+Si+Yt+ϵit,where Dit is a measure of opioid deaths in state i and year t, Mit indicates one or more cannabis policies, Xit are supplementary covariates, Si and Yt are vectors of state and year fixed effects, ϵit is the regression error term, and β^ provides the predicted impact of marijuana legalization. Prior studies have estimated

Data

We use National Vital Statistics System data from the Multiple Cause of Death (MCOD) files for the universe of U.S. deaths from 1999-2019. The MCOD provide information on a single underlying cause of death, up to 20 additional contributory causes, and demographic variables (Centers for Disease Control and Prevention 2022). Data are utilized here on four-digit International Classification of Diseases, Tenth Revision (ICD-10) mortality codes, state and county of residence, age, race/ethnicity,

Previous Findings are Not Robust to Specification Changes

We begin our empirical analysis by replicating and extending the results of four studies described above that use annual state-level data and DiD methods (Bachhuber et al. 2014; Powell, Pacula, and Jacobson 2018; Shover et al. 2019; Chan, Burkhardt, and Flyr 2020). We first employ specifications and data similar or identical to those originally used. Where possible, we obtained data directly from the papers, online supplements, or the authors. When this was not feasible, either because the

Legal Retail Marijuana Sales Raise Opioid Mortality

We next go beyond the extensions of previous studies just examined to provide an in-depth examination of the effects of cannabis legalization on opioid deaths over the 1999-2019 period, using the data and methods detailed in the Methods section.

Potential Problems with Difference-in-Differences Estimates

Recent literature identifies possible concerns with estimates obtained using standard DiD models when the timing of treatment implementation is staggered across locations (Goodman-Bacon 2021; Sun and Abraham 2021). In our context, the two most problematic issues are: 1) the usual procedure gives more weight to policy changes occurring in the middle of the analysis period than to those taking place near either end of it; and 2) the estimates may be biased if the treatment effects vary with the

Estimated Medical Marijuana Legalization Effects Change Over Time

Estimated medical cannabis legalization effects are sensitive to the period analyzed, with more beneficial or less detrimental consequences when analyzing data from 1999-2010 than when extending the sample to include later years. Some prior researchers (Smart 2015; Pacula and Smart 2017; Powell, Pacula, and Jacobson 2018) suggest that one cause may have been the Ogden memo, released in 2009, which deprioritized the federal prosecution of medical marijuana users and suppliers. They argue that

Illicit Fentanyl and the Effects of Marijuana Legalization

Next, we examine the possibility that cannabis legalization has become more harmful in recent years because of the growth of fentanyl in illicit drug markets. Fentanyl and its analogues began to increase opioid mortality in 2014, with a rapid acceleration after 2015 (Hedegaard et al. 2021). This closely accords with the timing of the worsening MML and MMD effects observed in the previous section.

Table 5 shows separate estimates from our preferred models for deaths from natural/semisynthetic

Heterogeneity

The effects of marijuana legalization may vary across population groups, depending on how increased access to cannabis changes overall opioid use and specific types of consumption. To examine this, we estimated our preferred models for population subgroups stratified by sex, race/ethnicity, and age. Table 6 summarizes the results. We rarely obtain strong evidence of differential impacts and caution against overinterpreting those we do observe, given the large number of models and coefficients

Discussion

There are many reasons why it may be desirable to legalize the use and sale of medical and recreational marijuana. Decreasing opioid mortality is not one of them. Some earlier research suggests that the legalization of medical cannabis reduces these deaths (Bachhuber et al. 2014), that it does so provided that there are sales through retail dispensaries (Powell, Pacula, and Jacobson 2018), or that it is recreational marijuana that has these benefits (Chan, Burkhardt, and Flyr 2020). However,

Uncited References

Hollingsworth, Wing and Bradford, Anderson

CRediT authorship contribution statement

Neil K. Mathur: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing – original draft, Writing – review & editing. Christopher J. Ruhm: Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Writing – original draft, Writing – review & editing.

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