This randomized Type 1 hybrid effectiveness-implementation trial (N=186) will evaluate the effectiveness and implementation of a peer-delivered problem solving and behavioral activation intervention for adherence to LAI-PrEP/ART (Peer Activate-LAI) compared to enhanced treatment as usual (ETAU) for a largely Black, substance-using population living with or at high risk for HIV. Specific aims are to: Aim 1: Evaluate the effectiveness of Peer Activate-LAI over 12-months on: a) LAI-PrEP/ART adherence (primary; receipt of all 6 maintenance injections within 7-day window); and b) substance use (secondary; WHOASSIST, urine toxicology); and c) Explore the moderating role of SRD-related factors (exploratory) Aim 2: To evaluate the implementation of Peer Activate-LAI including feasibility, acceptability, fidelity, and adoption guided by RE-AIM and Proctor's model,12,13 assessed using mixed methods, including a rapid ethnographic assessment of how SRD-related factors may affect implementation. Aim 3: To evaluate the economic viability of Peer Activate-LAI, including a) cost of implementation and sustainment, and b) cost-effectiveness from multiple stakeholder perspectives. This study will inform a potentially scalable, cost-effective model for facilitating effective adherence to LAI formulations of PrEP/ART within Black, substance-using populations with multiple minority identities who to date have had limited support for improving LAI adherence for HIV treatment and prevention.