Substance Use
Interpersonal Dynamics and Drug Injection Behaviors: A Longitudinal Study of Injecting Dyads Maia Scarpetta* Maia Scarpetta Scarpetta Scarpetta Scarpetta Scarpetta Scarpetta Scarpetta University of California San Francisco
Background: High-risk injecting behaviors often occur within ongoing injecting partnerships, yet longitudinal evidence on how within-partnership changes in interpersonal dynamics shape subsequent equipment sharing is limited. We examined whether month-to-month changes in partnership-level relationship factors predict later injecting risk.
Methods: We used longitudinal dyadic data from the UFO Partnership Study, in which verified injecting dyads were followed monthly for up to six months. The analytic sample included 92 individuals in 85 injecting partnerships, restricted to dyad-level visits. The primary outcome was dyad-mean recent sharing of needles and ancillary equipment. Interpersonal dynamics were measured using the Interpersonal Dynamics in Injecting Partnerships scale, with dyad-level exposures defined as the mean of partners’ scores for trust, intimacy, cooperation, risk perception, and power. Analyses used dyad-level repeated measures with cross-classified random effects for partnerships and individuals. Time-varying interpersonal dynamics were lagged to predict subsequent injecting risk, adjusting for baseline and time-varying covariates.
Results: Dyad-level trust varied over time and was positively associated with subsequent injecting risk (β = 0.40, 95% CI: 0.21–0.68). Power (β = 3.95, 95% CI: 2.24–5.64) and intimacy (β = −2.45, 95% CI: −4.38 to −0.51) showed nonmonotonic associations, with higher risk at intermediate power and at low and high levels of intimacy. Random-effects estimates indicated meaningful between-partnership heterogeneity. Cooperation and risk perception were not significantly associated with outcomes.
Conclusions: Changes in trust, power, and intimacy within injecting partnerships predict subsequent injecting risk. These findings underscore relationship dynamics as time-varying determinants of harm and potential targets for partnership-centered harm reduction and hepatitis C prevention.