The impact of severe mental illness on treatment retention and all-cause mortality of people in opioid agonist treatment

Background: Severe mental illness (SMI – psychotic and/or bipolar disorder) is common among individuals with opioid dependence. While we know that longer retention in opioid agonist treatment (OAT) is associated with reduced mortality, we don’t know whether these benefits extend to individuals with co-morbid SMI. In this study, we examine the impact of SMI on OAT retention in the index and first five treatment episodes and examine the impact of SMI on all-cause mortality.  

Methods: The cohort comprised of 14763 individuals receiving OAT for the first time in New South Wales, Australia, between 2006 and 2017. OAT records were linked to hospital, mental health, and custodial information. Extended Cox regression models and logistic generalised estimating equations were used to examine OAT retention and all-cause mortality. Adjustments for sex, age, year of treatment entry, indigenous status and treatment setting, geographical remoteness, socio-economic disadvantage and a history of criminal convictions, incarcerations, hospitalisations, mental health outpatient and emergency department presentations in the 12 months prior to OAT initiation. OAT exposure was modelled using a time-varying covariate (treatment).

Results: There were 1989 (13·5%) individuals with SMI in the cohort and 763 (6·2%) deaths during the follow-up period. The risk of treatment cessation of a first OAT episode was 16% higher for individuals with SMI [adjusted hazard ratio (aHR) 1·16, 95% CI 1·09-1·23]. Among those with multiple OAT episodes, differences in retention between those with SMI and without were most notable in the first three treatment episodes; this difference was attenuated between episodes four to five. The risk of all-cause mortality in treatment was two-fold higher in individuals diagnosed with SMI than in those without [aHR 2·30, 95% CI 1·62-3·26]. 

Conclusion: Strategies to increase retention in OAT for individuals with SMI may reduce mortality rates in this high-risk group. Improving retention rates could lead to a significant reduction in mortality rates among individuals diagnosed with comorbid SMI and opioid dependence. Interventions to support and engage individuals in OAT are needed early after commencing OAT.