People who inject drugs with sexual partners or close friends have high rates of syringe/ancillary equipment sharing and HIV and hepatitis C virus (HCV) infection. Although evidence suggests that interpersonal factors underlie these higher risk profiles, there is no quantitative measure of how interpersonal factors operate within injecting relationships. We aimed to develop and validate a quantitative scale to assess levels of injecting drug-related interpersonal factors associated with risky injecting behaviours within injecting partnerships.

We conducted qualitative interviews with 45 people who inject drugs (PWID) who reported having injecting partners to inform item development, and tested these items in a quantitative study of 140 PWID from San Francisco, USA, to assess internal reliability (Cronbach’s alpha) and validity (convergent, and discriminant validity).

With results from the qualitative interview data, we developed the Interpersonal Dynamics in Injecting Partnerships (IDIP) scale with 54 final items for 5 subscales of injecting-related interpersonal factors. Exploratory factor analysis revealed 5 factors (“trust”, “power”, “risk perception”, “intimacy”, and “cooperation”) with eigenvalues of 14.32, 6.18, 3.55, 2.46, and 2.14, explaining 57% of the variance, and indicating good internal reliability (alpha: 0.92–0.68). Strong convergent validity was observed in bivariate logistic regression models where higher levels of trust, intimacy, and cooperation within partnerships were positively associated with partners sharing needles and injecting equipment, whereas higher levels of power and risk perception were negatively associated with partners sharing needles and injecting equipment.

These findings offer strong evidence that the IDIP scale provides a psychometrically sound measure of injecting drug-related interpersonal dynamics. This measurement tool has the potential to facilitate additional investigations into the individual and collective impact of trust, intimacy, power, cooperation, and risk perception on injection drug using behaviours and engagement in HIV and HCV testing and treatment among PWID in a variety of settings.