This dissertation explores the lived experience of opiate substitution therapy (OST) patients in Ukraine. To complete this research, I conducted fourteen months of ethnographic research in OST programs across Ukrainebetween 2012 and 2014. I conducted extensive clinical observations and collected more than fifty interviews with patients and clinicians. The first chapter, the introduction, describes the historical and contemporary landscape of harm reduction and drug treatment services in Ukraine. The second chapter positions this research as a 'neurochemical ethnography, ' which taps into critical medical anthropology, phenomenology, and science studies to explore human-chemical relationships. The third chapter reveals that, in Ukraine, scientific utterances are also political utterances. This complicates the interpretation of evidence and destabilizes the epistemological foundation of 'evidence-based' practice. The fourth chapter shows that patients and clinicians in OST programs espouse conflicting explanatory models of addiction. This causes antagonism over the utility of OST drugs and the definition of treatment success. The fifth chapter analyses the semiotics of OST drugs and patients' active resistance to the clinical distinction between 'street drugs' and 'medicine.' The sixth chapter demonstrates that treatment-seeking decisions are motivated by patients' desire to embody normative social roles. They hope to achieve this goal by simplifying the logistics of their chemical self-management. In the seventh chapter, the conclusion, I outline practical recommendations for improving OST programs in Ukraine and elsewhere. In sum, this dissertation argues that clinical cultures and treatment-seeking behaviors are shaped by a 'somatic ethic, ' which not only governs discourses on drug use and addiction but also places social integration and acceptable personhood at odds with the practicalities of treatment.