BACKGROUND: Heroin injection is emerging as a significant dimension of the HIV epidemic in Kenya. Preventing transitions to injecting drug use from less harmful forms of use, such as smoking, is a potentially important focus for HIV prevention. There is, however, little evidence to support comprehensive programming in this area, linked to a shortage of analysis of the social and structural context for transitions, particularly in low-income settings. We explore accounts of transitions from smoking to injecting in Kenya to understand the role of individual, social and structural processes. METHODS: We combine data from two separate studies conducted in Kenya: an in-depth qualitative study of HIV care access for people who inject drugs (study 1) and an ethnographic study of the political economy of the heroin trade in Kenya (study 2). In-depth interviews with PWID and community observation from study 1 are triangulated with accounts from stakeholders involved in the heroin trade and documentary data from study 2. RESULTS: People who inject drugs link transitions to injecting from smoking to a range of social and behavioural factors, as well as particular aspects of the local drug supply and economy. We present these results in the form of two narratives that account for factors shaping transitions. A dominant narrative of 'managing markets and maintaining a high' results from a process of trying to manage poverty and a shifting heroin supply, in the context of deepening addiction to heroin. A secondary narrative focuses on people's curiosity for the 'feeling' of injecting, and the potential pleasure from it, with less emphasis on structural circumstances. CONCLUSIONS: The narratives we describe represent pathways through which structural and social factors interact with individual experiences of addiction to increase the risk of transitions to injecting. In response, HIV and harm reduction programmes need combinations of different strategies to respond to varied experiences of transitions. These strategies should include, alongside behaviour-oriented interventions, structural interventions to address economic vulnerability and the policing of the drug supply.