In the throes of an intractable overdose crisis, U.S. pharmacists have begun to engage in an unexpected practice—policing patients. Contemporary sociological theory does not explain why. Theories of professions and frontline work suggest professions closely guard jurisdictions and make decisions based on the logics of their own fields. Theories of criminal-legal expansion show that non-enforcement fields have become reoriented around crime over the past several decades, but past work largely focuses on macro-level consequences. This article uses the case of pharmacists and opioids to develop a micro-level theory of professional field reorientation around crime, the Trojan Horse Framework. Drawing on 118 longitudinal and cross-sectional interviews with pharmacists in six states, I reveal how the use of prescription drug monitoring programs (PDMPs)—surveillance technology designed for law enforcement but implemented in healthcare—in conjunction with a set of field conditions motivates pharmacists to police patients. PDMPs serve as Trojan horse technologies as their use shifts pharmacists’ routines, relationships with other professionals, and constructions of their professional roles. As a result, pharmacists route patients out of the healthcare system and leave them vulnerable to the criminal-legal system. The article concludes with policy recommendations and a discussion of future applications of the Trojan Horse Framework.