Commentary on Degenhardt et al. (2014): Access to opioid substitution therapy in prison is not enough-the crucial role of post-release retention in preventing drug-related harmsPrison opioid substitution therapy (OST) programmes have expanded considerably over the past decade in line with compelling evidence of the effectiveness of community-based OST programmes in reducing the health and social harms associated with injecting drug use [1]. Prison OST programmes have demonstrated utility in reducing heroin use, injecting and syringesharing in prison and increasing post-release treatment entry and retention [2]. But despite release from prison being identified as a particularly vulnerable period for drug-related mortality [3], there remains limited evidence of the impact of OST on post-release mortality [2].The paper by Degenhardt et al. [4] is therefore a welcome addition to the literature and represents a valuable contribution to informing evidence-based harm reduction responses in the criminal justice sector. The study makes the most of large, population-level data sets from one Australian jurisdiction, combined through data linkage, to examine the relationships between OST utilization, incarceration and mortality. Although these data are not without limitations, especially those related to unobserved confounding factors, the considerable population coverage and extensive retrospective time-period support the authors' contention that their findings represent 'unequivocal evidence' that providing prisoners with OST and, crucially, retaining them in OST as they transition into the community, will save lives.The impact of programme retention in reducing mortality is the most compelling finding reported by Degenhardt et al. Beyond simply making OST available, their findings highlight the importance of effective systems to retain people in OST as they transition from prison to the community and beyond. The more than eightfold reduced risk of drug-related mortality associated with OST retention in the immediate post-release period underscores the crucial role transitional OST programmes play in reducing drug-related harms. In this regard, it is encouraging that in more than 80% of incarceration episodes where OST was accessed, OST was also provided upon release. However, the high mortality risk in the initial post-release period also recommends other responses. Provision of naloxone with appropriate training [5], including through prison release programmes [6], is critical to reducing drug-related mortality in recently released prisoners [7]. Degenhardt et al. provide potentially useful data to inform the targeting of naloxone provision to those most at risk, including those who may face barriers to accessing and being retained on OST.While Degenhardt et al. replicate the immediate spike in post-release drug-related deaths demonstrated by others [3], it is important to note that most drug-related deaths in their cohort occurred after this 'high-risk period'. A focus on OST retention in the acute postrelease period ...