Background: Women who inject drugs (WWID) are disproportionately impacted by injection-related harms and substance use-related stigma. Guided by an integration of the Stigma and Substance Use Process Model and the Risk Environment Framework, this dissertation sought to further elucidate the impact of gender on substance use stigma and injection drug use trajectories. Methods: This dissertation includes a systematic review of substance use-related stigma (Chapter 2) and mixed methods analyses from three prospective cohorts of people who inject drugs (PWID) linked within the PReventing Injecting by Modifying Existing Responses (PRIMER) study. Cohorts provide data from San Diego, USA (n = 352) (Chapter 3), Vancouver, Canada (n = 1,739) (Chapters 3 & 4), and Tijuana, Mexico (n =531) (Chapters 3 & 4). Chapter 2 systematically reviews the scientific literature on the intersection of gender- and substance use- related stigma. Chapter 3, using a mixed methods approach, examines how gender influences the risk environment for processes of injection initiation. Chapter 4 applies discrete time survival analyses to assess the association between gender and the provision of first-time injection initiation assistance. Results: Of the 75 articles (Quantitative: n = 40; Qualitative: n = 35) included in Chapter 2, 23(57.5%) quantitative articles reported no association between gender and substance use- related stigma, whereas 34(97.1%) qualitative articles reported women experienced greater substance use stigma than men. Chapter 3 findings demonstrated a greater proportion of gender concordant (e.g., male-male vs. male-female) ‘assister’ and ‘assistee’ injection initiation pairs among PWID in Tijuana compared to Vancouver or San Diego. The gendered spatial risk environment of prisons/jails in Tijuana and social risk environments of intimate partnerships and caring in San Diego and Vancouver, respectively, likely explain differences in gender concordance of assister-assistee pairs by setting. Chapter 4 findings demonstrate that, compared to WWID, women were nearly 50% less likely to provide first-time injection initiation assistance in Tijuana, but not in Vancouver. Conclusions: These findings illustrate the need for intersectional approaches to research on the influence of gender on stigma- and injection drug use-related processes. Furthermore, these results can inform the development of critically needed gender- and site-specific injection prevention and intervention efforts.