BACKGROUND: Substance use stigma is a form of group-based exclusion, and delineating pathways from stigma to poor health requires a deeper understanding of the social dynamics of people who use drugs (PWUD). Outside of recovery, scant research has examined the role of social identity in addiction. Framed by Social Identity Theory/Self-Categorization Theory, this qualitative study investigated strategies of within-group categorization and differentiation among PWUD and the roles these social categories may play in shaping intragroup attitudes, perceptions, and behaviors. METHODS: Data come from the Rural Opioid Initiative, a multi-site study of the overdose epidemic in rural United States. We conducted in-depth interviews with people who reported using opioids or injecting any drug (n=355) living in 65 counties across 10 states. Interviews focused on participants' biographical histories, past and current drug use, risk behaviors, and experiences with healthcare providers and law enforcement. Social categories and dimensions along which categories were evaluated were inductively identified using reflexive thematic analysis. RESULTS: We identified seven social categories that were commonly appraised by participants along eight evaluative dimensions. Categories included drug of choice, route of administration, method of attainment, gender, age, genesis of use, and recovery approach. Categories were evaluated by participants based on ascribed characteristics of morality, destructiveness, aversiveness, control, functionality, victimhood, recklessness, and determination. Participants performed nuanced identity work during interviews, including reifying social categories, defining 'addict' prototypicality, reflexively comparing self to other, and disidentifying from the PWUD supra-category. CONCLUSION: We identify several facets of identity, both behavioral and demographic, along which people who use drugs perceive salient social boundaries. Beyond an addiction-recovery binary, identity is shaped by multiple aspects of the social self in substance use. Patterns of categorization and differentiation revealed negative intragroup attitudes, including stigma, that may hinder solidary-building and collective action in this marginalized group.