Changes in substance use behaviours across the prison journey: a systematic review of qualitative and quantitative evidence

Substance use-related problems are significantly overrepresented among prison populations internationally, with diagnosed substance use-related disorders affecting a substantial proportion of people in prison across high-income countries. This systematic review synthesised evidence on changes in substance use behaviours throughout prison and post-release transitions to inform harm reduction policy and practice. Following PRISMA guidelines, we searched six databases for peer-reviewed and grey literature published between 2014–2024. Studies examining substance use changes among adults at prison entry, during prison, and/or post-release were included. Two reviewers independently screened records and extracted data. Quality assessment used JBI Critical Appraisal Tools. Narrative synthesis was employed due to study heterogeneity. From 7,378 records, 34 studies met inclusion criteria (15 qualitative, 18 quantitative, 1 mixed methods). Included studies demonstrated moderate to high methodological quality, with notable gaps in reflexivity reporting and follow-up completeness. Many individuals ceased substance use during prison, with individual studies reporting cessation among around half or more of those using before custody, while harms such as needle sharing were concentrated among the smaller group who continued to use. Post-release represented a critical vulnerability window, with high resumption rates concentrated in the first days to weeks following release and qualitative evidence identifying the first 48 h as particularly high-risk for overdose. Age emerged as a key factor, with older adults more likely to report sustained cessation while younger individuals faced elevated risks throughout the prison journey. The prison journey involves both opportunities for change and periods of increased risk, with substance use declining for many while harms remain greater among those who continue to use. Critical vulnerability windows, particularly immediate post-release, offer targets for harm reduction. Findings highlight needs for prison-based harm reduction programmes, enhanced health-criminal justice integration, and age-responsive interventions. This systematic review was pre-registered on PROSPERO (CRD420250617547).