Within a backdrop of rising drug use in Kenya, this thesis applies the social ecology theory to elaborate the determinants of drug use trajectories and attendant HIV risks among women who inject drugs. This thesis comprises four parts. The first part is an overview of the epidemiology of injecting drug use. The second part is a literature review exploring 1) conceptualisation, 2) health impacts, and 3) determinants of drug-use trajectories. Based on a synthesis of 142 sources from four databases, literature suggests that the main phases of drug use consist of initiation, transition to injecting, and chronic addicted injecting. Additionally, drug use has significant negative impacts on the health of injectors. Although prior research has explored and identified micro- and macro-level determinants of injecting drug use, trajectory studies among women, particularly in sub-Saharan Africa, are rare. Based on this gap, the third part of this thesis explores determinants of drug use trajectories and HIV risks among 45 women, using secondary analysis of qualitative data. This study found that initiations, transitions, and chronic drug injecting, and the attendant HIV risks were not determined by a single factor but by multiple and intersecting social-ecological determinants located at the individual, interpersonal and societal-structural levels. By employing the social ecology theory, the importance of joblessness, gender inequity, economics, drug availability, and health systems in shaping trajectories and HIV risks was identified. The fourth part discusses these findings, and demonstrates that this study contributes to an empirical understanding of women’s drug use trajectories and HIV risks, advances scarce literature relating to qualitative secondary data analysis, and identifies the need to place intersectionality central to the future application of the social ecology theory. To limit women’s drug-injecting and attendant HIV risks, it is essential to focus on upstream social-structural determinants, alongside the currently emphasised individual and interpersonal determinants.