UCSF

Injection drug use drives human immunodeficiency virus (HIV) and hepatitis C virus (HCV) transmission through the sharing of needles and ancillary injection equipment. Most prevention studies focus on individual behavior, but injection-related risks occur within interpersonal contexts involving people who inject drugs (e.g., injecting partnerships). The Risk Environment Framework emphasizes that injection drug risks emerge from complex social and structural factors rather than individual choices alone, yet interpersonal dynamics within injecting partnerships remain understudied using dyadic methodologies.This dissertation comprises three complementary studies examining interpersonal dynamics within injecting partnerships using longitudinal dyadic data from San Francisco and Montreal (2015-2019) and validated Interpersonal Dynamics in Injecting Partnerships (IDIP) scale measurements. Study 1 asked which interpersonal factors are associated with equipment sharing, employing Actor-Partner Interdependence Models (APIM) with 136 partnerships to examine cross-sectional associations between five interpersonal factors (trust, power imbalance, risk perception, intimacy, and cooperation) and sharing behaviors. Study 2 asked whether interpersonal dynamics change within partnerships over time and predict subsequent risk behavior, using longitudinal dyadic modeling with 85 San Francisco partnerships followed monthly for up to six months. Study 3 asked whether injecting partnerships cluster into meaningful typologies, utilizing latent class mixed models with 69 partnerships to identify distinct patterns of interpersonal dynamics and their associations with partnership characteristics and outcomes.Study 1 found that actor-reported trust was positively associated with equipment sharing (β=0.41, 95% CI: 0.17-0.65), while both actor- and partner-reported power imbalance and risk perception were negatively associated with sharing. In HCV-serodiscordant partnerships, higher partner trust was associated with lower sharing. Study 2 demonstrated that within-partnership increases in dyad-level trust over time predicted higher subsequent equipment sharing (β=0.14, 95% CI: 0.02-0.26), with equipment sharing decreasing significantly over follow-up time. Study 3 identified two distinct partnership classes: Class 1 (23.2%) characterized by lower interpersonal connectedness but higher behavioral interdependence (87.5% cohabitation, 75.0% sexual relationships), and Class 2 (76.8%) with higher interpersonal connectedness but lower behavioral interdependence (49.1% cohabitation, 9.4% sexual relationships). Partnership type was significantly associated with cohabitation (OR=0.14, p=0.014) and sexual involvement (OR=0.03, p<0.001).This research advances understanding of interpersonal mechanisms driving injection-related risk behaviors by demonstrating that equipment sharing is shaped by mutual relational influence within partnerships rather than individual decisions alone. Trust emerges as a key dynamic that can both increase risk through normalization of sharing and potentially serve as a foundation for harm reduction interventions. The identification of distinct partnership typologies suggests that interventions may benefit from being to specific relationship characteristics rather than applying uniform approaches. These findings provide evidence for developing dyad-focused harm reduction interventions that acknowledge relational elements that acknowledge injecting partnerships as site of both risk and prevention.