State cannabis legalization and opioid use disorder in the US Veterans Health Administration, 2005 to 2019

Abstract: Aim: Cannabis use is legal in many U.S. states, and opioids may be substituted for cannabis. Fewer opioids may reduce its consequences, including opioid use disorder (OUD). We examined associations of medical and recreational cannabis law (MCL/RCL) enactment and changes in OUD prevalence among Veterans Health Administration (VHA) patients, and whether these trends differed by age (18-34, 35-64, 65-75). Methods: Using VHA electronic health records from 2005-2019 (∼4.3-5.6 million patients/year), we created yearly cross-sectional datasets and extracted ICD-9/10-CM OUD diagnoses. We calculated the adjusted yearly prevalence of OUD, controlling for continuous age, sex, race, ethnicity, and time-varying state covariates. We then used staggered-adoption difference-in-difference analyses to estimate the effect with 95% confidence interval (CI) of MCL and RCL enactment on changes in OUD prevalence, accounting for the year that state laws were enacted and covariates. Results: From 2005-2019, adjusted OUD prevalence decreased from 1.09% to 1.05% in states without cannabis laws, and increased from 1.13% to 1.31% and 1.20% to 1.22% in MCL and RCL states, respectively. MCL-only enactment was associated with a 0.10% (CI:0.09-0.12, p <0.001) absolute increase in OUD prevalence, while RCL enactment was associated with a 0.08% (CI:0.70-0.10, p <0.001) absolute increase in OUD prevalence. The effect of MCL and RCL on OUD prevalence was greatest in patients aged 65-75 years, with an absolute increase of 0.09% (CI:0.08-0.10, p <0.001) and 0.22% (CI:0.20-0.23, p <0.001), respectively. In veterans age 18-34 and 35-64, MCL enactment resulted in a more modest increase in OUD prevalence (age 18-34: absolute increase=0.08%, CI: 0.04-0.12, p <0.001; age 35-64: absolute increase=0.07%, CI: 0.06-0.09, p <0.001), while RCL was not associated with OUD. Conclusions: Over time, OUD was disproportionately prevalent in patients residing in states with MCL, with further increases in OUD among older adults who also resided in states with RCL. These findings contradict the notion that access to legal cannabis reduces opioid-related harms.

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