PROJECT ABSTRACT: Rising rates of injection drug use in the United States have led to a crisis of serious injection-related infections (SIRI) affecting people who inject drugs (PWID), such as endocarditis, osteomyelitis, and spinal abscess. SIRI are associated with high severity and mortality, often needing long hospitalizations, and need for intensive health services. Addiction treatment interventions, such as addiction specialty consultation or initiation of medications for opioid use disorder, have shown benefit in smaller studies and in commercially insured populations. Evidence generated in Medicaid populations is needed to inform Medicaid program policies. In this proposal, our overall objective is to comprehensively understand addiction treatment delivery for patients with SIRI, including variation and disparities in treatment intervention delivery, and the associations of those interventions with clinical and economic outcomes. Specifically, we aim to 1) Use national Medicaid administrative data from 2015-21 to determine if delivery of addiction treatment interventions to PWID hospitalized with SIRI is associated with clinical and economic outcomes, 2) how hospital- and area-level capacity to provide addiction treatment explains the variation in treatment delivery, and 3) use qualitative methods to determine the factors that promote or hinder addiction treatment interventions in hospital settings. To improve outcomes for PWID with SIRI, we first need to understand the current landscape of healthcare delivery, and the motivating factors that will transform healthcare practices to better meet the needs of PWID. Understanding the gaps in evidence-based healthcare delivery for SIRI in the United States are critical first steps for implementing system change to improve care for PWID with SIRI.