Abstract
Abstract Prisons are central institutions in the criminal justice system, yet limited data have impeded a systematic analysis of their internal dynamics. We compile a novel, comprehensive dataset covering all prisons in England and Wales to evaluate the effects of a UK-wide ban on new psychoactive substances—potent synthetic drugs designed to circumvent regulation and readily trafficked into custodial settings. We first document the diffusion of these substances across the prison system and a strong correlation with rising violence. Using a difference-in-differences strategy, we find that the ban produced a sustained reduction in drug availability, but also a short-term increase in both interpersonal violence and self-harm among inmates. These findings underscore the complex trade-offs involved in supply-side interventions within high-risk environments. Effective policy responses in such settings should integrate demand-side measures—such as addiction treatment and mental health services—to mitigate unintended harms and improve institutional safety.