Examining the gender composition of drug injecting initiation events: A mixed methods investigation of three North American contexts

•We compared the gender composition of injection initiation events across contexts.•A greater proportion of injection initiation events were gender concordant in Tijuana.•Prisons and jails may contribute to gender concordant initiation events in Tijuana.•Intimate partnerships may contribute to gender discordant events in San Diego.•Caring for others in an overdose crisis may explain the discordance in Vancouver. Gender influences the health and social risks faced by individuals initiating drug injecting. Using mixed methods across three settings in North America, we investigated the gender composition of injection initiation events and the gendered risk environments in which they occurred. The PReventing Injecting by Modifying Existing Responses (PRIMER) study pooled data from three prospective community-recruited cohorts of people who inject drugs (PWID) in San Diego, USA, Vancouver, Canada, and Tijuana, Mexico. A qualitative subsample provided narrative data on their experiences of, and the contexts for, injection initiation events. Guided by Rhodes’ risk environment framework, we examined the gender composition of initiation events stratified by city, and analyzed qualitative data using abductive thematic analyses. Among 2,622 PWID (Tijuana: n = 531; San Diego: n = 352; Vancouver: n = 1,739), 112 (4.3%) reported providing initiation assistance to injection-naïve individuals in the previous six months. The proportion of gender concordant (e.g., male-male) initiation pairs varied, (χ2 = 10.32, p