Medicaid expansion and opioid overdose mortality among socioeconomically disadvantaged populations in the US: A difference in differences analysis.

Background: Opioid-related overdoses are a major cause of mortality in the US. Medicaid Expansion is posited to reduce opioid overdose-related mortality (OORM), and may have a particularly strong effect among people of lower socioeconomic status. This study assessed the association between state Medicaid Expansion and county-level OORM rates among individuals with low educational attainment.

Methods: This quasi-experimental study used lagged multilevel difference-in-difference models to test the relationship of state Medicaid Expansion to county-level OORM rates among people with a high-school diploma or less. Longitudinal (2008-2018) OORM data on 2978 counties nested in 48 states and the District of Columbia (DC) were drawn from the National Center for Health Statistics. The state-level exposure was a time-varying binary-coded variable capturing pre- and post-Medicaid Expansion under the Affordable Care Act (an "on switch"-type variable). The main outcome was annual county-level OORM rates among low-education adults adjusted for potential underreporting of OORM.

Findings: The adjusted county-level OORM rates per 100,000 among the study population rose on average from 10.26 (SD = 13.56) in 2008-14.51 (SD = 18.20) in 2018. In the 1-year lagged multivariable model that controlled for policy and sociodemographic covariates, the association between state Medicaid Expansion and county-level OORM rates was statistically insignificant.

Conclusions: We found no evidence that expanding Medicaid eligibility reduced OORM rates among adults with lower educational attainment. Future work should seek to corroborate our findings and also identify - and repair - breakdowns in mechanisms that should link Medicaid Expansion to reduced overdoses.