BACKGROUND: Severe mental illness (SMI - psychotic and bipolar disorders) is common among individuals with opioid use disorder (OUD). This study examined the impact of SMI on opioid agonist treatment (OAT) retention, and SMI and OAT on all-cause mortality.

METHODS: A retrospective cohort study of 14763 individuals receiving OAT for the first time in New South Wales, Australia, 2006-2017. OAT records were linked to hospital, mental health treatment, and custodial information. Multivariable Cox regression models were used to compare OAT cessation/retention and all-cause mortality among those with/without SMI, adjusting for potential confounders.

RESULTS: There were 1989 (13.5%) individuals with SMI and 763 (5.2%) deaths. The risk of treatment cessation during a first OAT episode was 16% higher for individuals with SMI [adjusted hazard ratio (aHR) 1.16, 95% confidence interval (CI) 1.09-1.23]. Among those with multiple OAT episodes, differences in retention between those with/without SMI were most notable in the first three treatment episodes; this difference was attenuated between episodes four to five. Although SMI increased mortality risk [aHR 1.35; 95% CI 1.14-1.60] and being in OAT decreased mortality risk [aHR 0.24; 95% CI 0.20-0.29], OAT status did not modify the effect of SMI on overall mortality risk.

CONCLUSIONS: Comorbid SMI and OUD significantly reduces OAT retention during early episodes of treatment and is associated with increased mortality risk. There is no differential treatment effect on mortality among those with and without SMI. Interventions to identify, support, and engage individuals with SMI in OAT are needed early after commencing treatment.