INTRODUCTION AND AIMS: Naloxone is an opioid antagonist used for emergency resuscitation following opioid overdose. Prisoners with a history of heroin use by injection have a high risk of drug-related death in the first weeks after prison-release. The N-ALIVE trial was planned as a large prison-based randomised controlled trial (RCT) to test the effectiveness of naloxone-on-release in the prevention of fatal opiate overdoses soon after release. The N-ALIVE pilot trial was conducted to test the main trial's assumptions on recruitment of prisons and prisoners, and the logistics for ensuring that participants received their N-ALIVE pack on release. DESIGN AND METHODS: Adult prisoners who had ever injected heroin, were incarcerated for ≥7 days and were expected to be released within 3 months were eligible. Participants were randomised to receive, on liberation, a pack containing a single 'rescue' injection of naloxone or a control pack with no naloxone syringe. The trial was double-blind prior to prison-release. RESULTS: We randomised 1685 prisoners (842 naloxone; 843 control) across 16 prisons in England. We stopped randomisation on 8 December 2014 because only one-third of administrations of naloxone-on-release were to the randomised ex-prisoner; two-thirds were to others whom we were not tracing. DISCUSSION AND CONCLUSIONS: Prevention RCTs are seldom conducted within prisons; we demonstrated the feasibility of conducting a multi-prison RCT to prevent fatality from opioid overdose in the outside community. We terminated the N-ALIVE trial due to the infeasibility of individualised randomisation to naloxone-on-release. Large RCTs are feasible within prisons.