PROJECT SUMMARY Individuals with opioid use disorder (OUD) who are involved with the criminal legal system (i.e., incarcerated, arrested) face an increased risk for morbidity and mortality, and few receive evidence-based treatment while incarcerated. The two most commonly used medications for OUD (MOUD), buprenorphine and methadone, are the most effective treatment interventions that can be provided during incarceration and post-release. It is unknown if the onset of ubiquitous fentanyl use has altered the use of MOUD type among individuals with OUD during incarceration, whether these potential changes in MOUD use additionally impact post-release health outcomes, and how these trajectories may differ by individual factors. This F31 will use a convergent mixed- methods design to address two research aims: (1) Analyze a comprehensive longitudinal database on health service use and health outcomes among individuals who received buprenorphine or methadone during incarceration at a unified jail and prison system in the Northeast (n=4558) from 2016 to 2023. Specifically, a retrospective cohort design will be used to examine predictors and uses of buprenorphine and methadone during incarceration and post-release, and the association between buprenorphine and methadone use during incarceration and post-release health outcomes (treatment retention, mortality) will be investigated over this time period. (2) Conduct and analyze one-on-one, in-depth interviews with individuals released from incarceration (n=30; 12 received methadone, 12 received buprenorphine, 6 received both) to identify individual, interpersonal, community, and structural factors that may shape the selection of MOUD type during and upon release from incarceration. This data will be used to examine preferences for MOUD delivery and type, experiences with MOUD delivery while incarcerated, and intentions to use MOUD post-release. Data from both aims will be integrated to fully contextualize the uses of health services and outcomes among this population. The proposed research addresses an urgent need to reduce morbidity and mortality surrounding OUD and fentanyl use among criminal legal involved populations by understanding the rapidly evolving opioid public health crisis and outcomes associated with evidence-based treatments. This two-year training fellowship will support the applicant’s development of content expertise, and knowledge and application of advanced longitudinal statistical methods, mixed-methods data collection and analysis, and scientific dissemination. Completion of this study will provide preliminary data for a postdoctoral NIH grant application exploring adaptations to the carceral-based provision of evidence-based OUD treatment. The extensive training this mentorship team provides will prepare the applicant for a successful career as an independent addiction science researcher.