The relationship of medical and recreational cannabis laws with opioid misuse and opioid use disorder in the USA: Does it depend on prior history of cannabis use?

Background: Wider availability of cannabis through medical and recreational legalization (MCL alone and RCL+MCL) has been hypothesized to contribute to reductions in opioid use, misuse, and related harms. We examined whether state adoption of cannabis laws was associated with changes in opioid outcomes overall and stratified by cannabis use. Methods: Using National Survey on Drug Use and Health (NSDUH) data from 2015 to 2019, we estimated cannabis law associations with opioid (prescription opioid misuse and/or heroin use) misuse and use disorder. All logistic regression models (overall models and models stratified by cannabis use), included year and state fixed effects, individual level covariates, and opioid-related state policies. Stratified analyses were restricted to individuals who reported lifetime cannabis use prior to law adoption to reduce potential for collider bias. Estimates accounted for multiple comparisons using false discovery rate (FDR) corrections and sensitivity to unmeasured confounding using e-values. Results: Overall, MCL and RCL adoption were not associated with changes in the odds of any opioid outcome. After restricting to respondents reporting past-year cannabis use, we observed decreased odds of past year opioid misuse (adjusted odds ratio [AOR]: 0.57 [95 % confidence interval [CI]: 0.38, 0.85]; FDRp-value: 0.07), among individuals in states with MCL compared to those in states without cannabis laws. RCLs were not associated with changes in the odds of any opioid outcome beyond MCL adoption. Conclusion: Comparing individuals in MCL alone states to those in states without such laws, we found an inconsistent pattern of decreased odds of opioid outcomes, which were more pronounced among people reporting cannabis use. The pattern did not hold for individuals in RCL states. In line with a substitution-oriented perspective, findings suggests that MCLs may be associated with reductions in opioid use among people using cannabis but additional work to replicate and expand on these findings is needed.