Project Summary Fatal overdose is a major cause of mortality among people who use opioids (PWUO). Yet, fatal overdose can be prevented when naloxone is promptly administered. Bystanders of an overdose are well positioned to administer naloxone and can include both people who use and do not use drugs. In our prior work, we have shown that PWUO can be peer educators (PEs) for overdose prevention. In previous interventions we have focused on training network members who use drugs to respond to an overdose. Yet, pilot data suggests that overdoses are also often witnessed by people who do not use drugs. Drug user stigma may impact PWUO engaging in conversations about overdose prevention with non-using networks as well as non-using network members’ willingness to always carry naloxone. Based on pilot data, we propose expanding an overdose peer education intervention to focus on non-using network members and to address drug user stigma. The intervention seeks to prepare network members to respond to an overdose and normalize discussion about overdose prevention planning between PWUO and their network members. Using an RCT study design, 300 index PWUO will be recruited along with 450 network members. The indexes will be randomly assigned to the (1) standard of care (SOC) or (2) an experimental peer education condition (PEC). The proposed design allows examination of the effectiveness of the intervention to train non-using network members and enhance communications among PWUO and their network members. We will use mHealth to assess geographic coverage of peer education and geospatially situated stigma in order to understand factors associated with the geospatial coverage of the PEs. The RE-AIM framework will guide the collection of qualitative interview data to identify barriers and facilitators to intervention implementation. Participants will be assessed every 6 months for 2 years. The proposed design also will allow for the longitudinal examination of pathways between social and psychological, and mental health factors and overdose behaviors which can guide future interventions. Aim 1: Implement and evaluate an equal attention 2-group RCT PWUO peer educator intervention that focuses on training network members, especially non-drug using network members, to encourage overdose prevention and response behaviors. Aim 2: Assess factors associated with geospatial coverage of peer outreach and geospatially/socially situated drug user stigma using EMA/mHealth.