Objective: To determine the effect of Medicaid expansion on the use of opioid agonist treatment for opioid use disorder (OUD) and to examine heterogeneous effects by provider supply and Medicaid acceptance rates.

Data sources: Yearly state-level data on methadone dispensed from opioid treatment programs (OTPs), buprenorphine dispensed from OTPs and pharmacies, number of OTPs and buprenorphine-waivered providers, and percent of OTPs and physicians accepting Medicaid.

Study design: This study used difference-in-differences models to examine the effect of Medicaid expansion on the amount of methadone and buprenorphine dispensed in states between 2006 and 2017. Interaction terms were used to estimate heterogeneous effects. Sensitivity analyses included testing the association of outcomes with Medicaid enrollment and state insurance rates.

Principal findings: The estimated effects of Medicaid expansion on buprenorphine and methadone dispensed were positive but imprecise, meaning we could not rule out negative or null effects of expansion. The estimated associations between state insurance rates and dispensed methadone and buprenorphine were centered near zero, suggesting that improvements in health coverage may not have increased OUD treatment use. The effect of Medicaid expansion was larger in the states with the most waivered providers compared to states with the fewest waivered providers. In the states with the most waivered providers, the average estimated effect of expansion on buprenorphine dispensed was 12 kg/y, enough to treat about 7500 individuals. We did not find evidence that the effect of expansion was consistently modified by OTP concentration, OTP Medicaid acceptance, or physician Medicaid acceptance.

Conclusions: Gains in health coverage may not be sufficient to increase OUD treatment, even in the context of high treatment need. Provider capacity likely limited Medicaid expansion's effect on buprenorphine dispensed. Policies to increase buprenorphine providers, such as ending the waiver requirement, may be needed to ensure coverage gains translate to treatment access.