BACKGROUND Overdose is the leading cause of death for people released from prison, and opioid agonist treatment is associated with reductions in mortality after imprisonment. However, few studies have explored the interplay of the potential modifiable risk factors and protective factors for mortality after release from prison. We aimed to describe all-cause mortality and overdose mortality among individuals released from Norwegian prisons during 2000-22 and to identify pre-existing risk factors associated with both types of mortality among these individuals for 6 months.

METHODS For this prospective analysis, we used data from the Norwegian Prison Release Study (nPRIS), which includes all people in prison in Norway between Jan 1, 2000, and Dec 31, 2022; the Norwegian Cause of Death Registry; the Norwegian Prison Registry; the Norwegian Patient Registry; and Statistics Norway. All prisons in Norway that were open during this period were included. People who did not have a Norwegian personal identification number or were serving their sentence outside of prison units were excluded from this analysis. To identify pre-existing risk factors associated with all-cause and overdose mortality among people released from prison, we left-censored the observation period on Jan 1, 2010, creating a subsample of individuals. We calculated crude mortality rates (CMRs) and corresponding 95% CIs as the number of deaths per 100 000 person-years for several time periods after release. The primary outcomes were all-cause mortality and overdose mortality according to the ICD-10, assessed in all participants and analysed via two separate Cox proportional-hazards models.

FINDINGS The total nPRIS cohort included 112 877 individuals released from prison in Norway between 2000 and 2022, 11 995 (10·6%) of whom were female and 100 865 (89·4%) of whom were male. We identified 13 004 instances of all-cause mortality and 3085 instances of overdose mortality during the 1 463 035 person-years. The estimated CMR for all-cause mortality was 889 (95% CI 874-904) per 100 000 person-years and for overdose mortality was 211 (203-218) per 100 000 person-years. Among people diagnosed with opioid use disorder before entering prison during 2010-22 (n=6830), provision of opioid agonist treatment was estimated to be associated with reductions in both all-cause mortality (hazard ratio 0·58, 95% CI 0·39-0·85) and overdose mortality (0·51, 0·31-0·82) in the 6 months after leaving prison after adjustment for sociodemographic, prison-related, and clinical characteristics.

INTERPRETATION In people diagnosed with opioid use disorder released from Norwegian prisons, opioid agonist treatment provided while in prison was a protective factor for both all-cause and overdose mortality at 6 months. Provision of opioid agonist treatment while in prison is crucial in reducing mortality for 6 months after release and should be available to all people in prison who have treatment needs.

FUNDING South-Eastern Norway Regional Health Authority and the Research Council of Norway.