Opioid medication treatment access is a public health priority aimed to improve opioid use treatment outcomes. However, Medicaid does not cover all forms of MOUD, particularly methadone, in many states. We examined associations between medication for opioid use disorder (MOUD) plans and substance use treatment discharge reason (e.g., completed treatment, dropped out of treatment) as well as treatment retention (i.e., length of stay), and estimated whether these relationships were modified by state Medicaid methadone coverage. Data from the 2016 Treatment Episode Data Set for Discharges (TEDS-D) included 152,196 opioid-related treatment episodes from 47 states using relative risk regression with state clustering. Discharges involving MOUD had higher treatment retention for >180 days (aRR: 1.60, 95% CI: 1.29, 1.99) and >365 days (aRR: 2.64, 95% CI: 2.00, 3.49) but lower treatment completion (aRR: 0.46, 95% CI: 0.38, 0.57). There was no evidence that state Medicaid methadone coverage modified any of these relationships. Focusing on treatment completion alone may obscure health benefits associated with longer MOUD treatment retention.