PROJECT SUMMARY/ABSTRACT Drug overdose deaths continue to rise in the United States, accounting for over 100,000 deaths in 2021 alone. Methamphetamine and opioid co-use are largely fueling the most recent increase in mortality. In addition, co- use associated hospital admissions increased more than five-fold between 2003 and 2015. Previous research and hospital-based addiction services for substance use have focused on opioid use alone. These addiction medicine services have increased rates of medication for opioid use disorder and improved patient linkage to continuing outpatient treatment. As a result, hospital length of stay and readmission rates have decreased. However, past research indicates individuals with co-use have different sociodemographic and health profiles and are less likely to receive medication for opioid use disorder than individuals who use opioids alone. There is limited evidence of how existing hospital-based addiction services effect patients who co-use methamphetamine and opioids or how these services could be tailored to better treat hospitalized patients with co-use. The overall objective of this Kirschstein-NRSA F30 fellowship is to generate evidence that can guide the eventual development, implementation, and evaluation of hospital addiction medicine treatment services tailored to co-use. The specific aims of this proposal are: 1) to characterize patient demographics and hospitalization characteristics associated with co-use from a nationally representative sample of inpatient hospitalizations, 2) to quantify the risk of re-admission and mortality following hospital discharge, by substance use profile, and whether differences vary by the receipt of hospital-based addiction services and 3) to identify barriers and facilitators to providing hospital-based addiction services tailored for patients with co-use. The proposed mixed methods project is based on a sequential explanatory approach. It includes training in both quantitative (Aims 1 & 2) and qualitative (Aim 3) research methodologies. Through this work and a thoughtfully designed training plan, the trainee will achieve the following goals: 1) develop health services research expertise, 2) advance the substance use research field as it relates to polysubstance use, 3) integrate research, clinical, and advocacy activities in the field of addiction medicine. This supports his long-term goal of bridging health services research and clinical medicine to support patients with substance use disorders as a physician-researcher.