Abstract
BACKGROUND: The role of cannabis reform in shaping North America's overdose and addiction crisis remains hotly contested. People who inject drugs (PWID) sometimes substitute cannabis for opioids. Yet, no research has examined the effects of cannabis legalization on opioid use among PWID-- despite major potential for PWID to benefit from policy interventions reducing opioid-related harms. We examined whether legalizing cannabis for medical use (MCL) vs. both MCL and adult/recreational use (MCL+RCL) was associated with changes in substance use among PWID, overall and by sex and race/ethnicity. METHODS: This serial cross-sectional observational study used staggered adoption difference-in-differences models to analyze data (2012, 2015, 2018, 2022) from 28,069 PWID from 13 states. We examined associations between time-varying implementation of MCLs and MCL+RCLs and self-reported past 12-month daily use of cannabis and non-medical opioid use. RESULTS: Compared to MCL, MCL+RCL was associated with a 9-11 % decrease in the probability of daily opioid misuse (95 % CIs: [any opioids: -14.0, -4.0]; [injected opioids, -19.0, -2.0]). The probability of daily cannabis use increased from 15 % to 20 % for non-Latinx White PWID in states transitioning from no legalization to MCL (95 % CIs: 14.0, 17.0; 17.0, 23.0). CONCLUSIONS: Cannabis legalization may shape daily opioid consumption among PWID, potentially reducing drug-related harms. Differences in cannabis use following legalization may reflect disparate impact by race, due to structural racism or other factors. Future research examining whether policy attributable changes in substance use manifest health benefits among PWID is critical to developing evidence- based cannabis reform.