A long-standing policy prohibits the use of federal funds for Medicaid services in Institutions for Mental Diseases (facilities with more than sixteen beds that specialize in mental health or substance use disorder treatment). Beginning in 2015, states could apply for Section 1115 Medicaid waivers, which permit federal funding for Institutions for Mental Diseases services and require improvements in opioid use disorder (OUD) treatment. Using 2016-20 Medicaid data, we compared changes in the use of medications for OUD and nonfatal overdoses in seventeen states with waivers approved during 2017-19 to changes in eighteen states without waivers. Waiver implementation was not associated with improvements in overall medication treatment, buprenorphine and naltrexone prescribing, or rates of nonfatal overdoses among Medicaid enrollees with OUD. Waiver implementation was associated with a 2.3-percentage-point increase in the use of methadone with waiver implementation, as a result of coverage changes, and a 3.7-percentage-point increase in any medication treatment among Medicaid enrollees diagnosed with severe OUD who had an inpatient or residential stay. Our findings suggest that such waivers adopted by states during 2017-19 were not associated with significant improvements in medication treatment or reductions in nonfatal opioid-related overdoses among Medicaid enrollees with OUD. However, they may have moderately improved the use of medication treatment for those with severe OUD.