Aims Social norms are a key source of influence on health behavior s. This study examined changes in social normsand relationships between HIV injection risk behaviors and social norms among injection drug users (IDUs) involved inan experimental intervention. Design Randomized clinical trial. Setting An HIV Prevention Trials Network study,Philadelphia, USA. Participants IDUs, called indexes, and their social network members, who were drug or sexpartners, were recruited for an HIV prevention intervention and followed for up to 30 months (n = 652). Indexes wererandomized into a peer education intervention or control condition. Measurements Outcomes of injection-relatedHIV risk behaviors (sharing needles, sharing cookers, sharing cotton, front-/back-loading) were measured every6 months and the social norms of these four risk behaviors were assessed every 12 months. Findings There wasa statistically significant intervention effect on all four social norms of injection behaviors, with participants in theintervention reporting less risky social norms compared with controls (changes in mean score: needles, -0.24,P = 0.007; cookers, -0.33, P = .004; cottons, -0.28, P = .0165; front-/back-loading, -0.23, P = .002). There was alsoa statistically significant bidirectional association with social norms predicting injection risk behavior s at the nextassessment and risk behaviors predicting social norms at the subsequent visit. Conclusions Through social networkinterventions it is feasible to change both injection risk behaviors and associated social norms. However, it is criticalthat social network interventions focus on publically highlighting behavior changes, as changing social norms withoutawareness of behaviors change may lead to relapse of risk behaviors.