BackgroundPrisons had partial exemption from the UK’s 2006/7 smoking bans in enclosed public spaces. They became one of the few workplaces with continuing exposure to second-hand smoke, given the high levels of smoking among people in custody. Despite the introduction of smoke-free prisons elsewhere, evaluations of such ‘bans’ have been very limited to date.ObjectiveThe objective was to provide evidence on the process and impact of implementing a smoke-free policy across a national prison service.DesignThe Tobacco in Prisons study was a three-phase, multimethod study exploring the periods before policy formulation (phase 1: pre announcement), during preparation for implementation (phase 2: preparatory) and after implementation (phase 3: post implementation).SettingThe study was set in Scotland’s prisons.ParticipantsParticipants were people in custody, prison staff and providers/users of prison smoking cessation services.InterventionComprehensive smoke-free prison rules were implemented across all of Scotland’s prisons in November 2018.Main outcome measuresThe main outcome measures were second-hand smoke levels, health outcomes and perspectives/experiences, including facilitators of successful transitions to smoke-free prisons.Data sourcesThe study utilised cross-sectional surveys of staff (total,n = 3522) and people in custody (total,n = 5956) in each phase; focus groups and/or one-to-one interviews with staff (n = 237 across 34 focus groups;n = 38 interviews), people in custody (n = 62 interviews), providers (n = 103 interviews) and users (n = 45 interviews) of prison smoking cessation services and stakeholders elsewhere (n = 19); measurements of second-hand smoke exposure (e.g. 369,208 minutes of static measures in residential areas at three time points); and routinely collected data (e.g. medications dispensed, inpatient/outpatient visits).ResultsMeasures of second-hand smoke were substantially (≈ 90%) reduced post implementation, compared with baseline, largely confirming the views of staff and people in custody that illicit smoking is not a major issue post ban. Several factors that contributed to the successful implementation of the smoke-free policy, now accepted as the ‘new normal’, were identified. E-cigarette use has become common, was recognised (by both staff and people in custody) to have facilitated the transition and raises new issues in prisons. The health economic analysis (lifetime model) demonstrated that costs were lower and the number of quality-adjusted life-years was larger for people in custody and staff in the ‘with smoke-free’ policy period than in the ‘without’ policy period, confirming cost-effectiveness against a £20,000 willingness-to-pay threshold.LimitationsThe ability to triangulate between different data sources mitigated limitations with constituent data sets.ConclusionsTo our knowledge, this is the first study internationally to analyse the views of prison staff and people in custody; objective measurements of second-hand smoke exposure and routine health and other outcomes before, during and after the implementation of a smoke-free prison policy; and to assess cost-effectiveness. The results are relevant to jurisdictions considering similar legislation, whether or not e-cigarettes are permitted. The study provides a model for partnership working and, as a multidimensional study of a national prison system, adds to a previously sparse evidence base internationally.Future workPriorities are to understand how to support people in custody in remaining smoke free after release from prison, and whether or not interventions can extend benefits to their families; to evaluate new guidance supporting people wishing to reduce or quit vaping; and to understand how prison vaping practices/cultures may strengthen or weaken long-term reductions in smoking.Study registrationThis study is registered as Research Registry 4802.FundingThis project was funded by the National Institute for Health Research (NIHR) Public Health Research programme and will be published in full inPublic Health Research; Vol. 10, No. 1. See the NIHR Journals Library website for further project information.