The current study describes an individual participant data meta-analysis (IPDMA) testing the efficacy of a peer-network counseling (PNC) intervention for preventing substance use escalation in adolescents and young adults. PNC has shown efficacy in reducing substance use among adolescents and young adults across small-scale randomized controlled trials (RCTs). Identifying expected large-scale effects and moderators is an important next step in guiding use of PNC in practice. To this end, we combine three small-scale RCTs to test PNC intervention effects on substance use change in a combined sample of 421 adolescents and young adults (50% intervention, 55% female, 69% Black/African-American, M age [SD] = 17.3 [2.2] years). Our approach combines latent change score modeling in a structural equation modeling (SEM) framework with study-level fixed effects to obtain (a) a more generalizable PNC effect than we could obtain with each constituent sample and (b) greater power and precision for individual-level moderation of treatment effects. We found that although PNCg main effects on substance use outcomes (past 30-day cannabis, alcohol, tobacco, and drug use) were not significant, PNC effects were moderated by individual-level pre-intervention substance use frequency. PNC more strongly reduced drug use at the 1-month follow-up and cannabis use at the 3-month follow-up among participants who showed higher baseline use of these substances. Implications of our approach and findings for prevention researchers are discussed.