Dyadic interactions and injecting practices: Unpacking the role of interpersonal dynamics in drug injection partnerships.

BACKGROUND: High-risk injecting behaviors often occur in the context of injecting partnerships, yet how interpersonal (dyadic) dynamics shape individual injecting practices remains poorly characterized. METHODS: Baseline cross-sectional data from 131 injecting partnerships (262 dyad-member observations) in San Francisco and Montreal were analyzed. Injecting-related interpersonal factors (trust, power imbalance, risk perception, intimacy, and cooperation) were measured using the validated 54-item Interpersonal Dynamics in Injecting Partnerships (IDIP) scale. Actor-Partner Interdependence Models (APIM), implemented via linear mixed-effects models, estimated the independent effects of each partner's IDIP scores on equipment sharing while accounting for dyadic interdependence. RESULTS: Overall, 74% reported recent equipment sharing. After covariate adjustment, actor-reported trust was positively associated with equipment sharing (β = 0.41, 95% CI: 0.17-0.65). Both actor (β = -0.50, 95% CI: -0.84 to -0.17) and partner (β = -0.51, 95% CI: -0.82 to -0.20) reports of power imbalance, and both actor (β = -0.28, 95% CI: -0.51 to -0.04) and partner (β = -0.23, 95% CI: -0.45 to -0.001) reports of risk perception, were associated with decreased sharing. In HCV-serodiscordant dyads, higher partner-reported trust was associated with lower sharing (interaction β = -0.56, 95% CI: -0.99 to -0.12); no other moderation effects were significant. CONCLUSIONS: Trust, power imbalance, and risk perception were associated with injecting equipment sharing, with a protective effect of partner trust most pronounced in HCV-serodiscordant dyads. Harm reduction strategies that leverage dyadic dynamics, including partner-based HCV testing and dyadic counseling, may strengthen HIV and HCV prevention, particularly when tailored to partnership composition.