The impact of severe mental illness on treatment retention and all-cause mortality of people in opioid agonist treatment: A retrospective data linkage study in New South Wales.

Author: Nicola Jones, Natasa Gisev, Julia Lappin, Rachel Deacon, Llewellyn Mills, Nick Lintzeris, Sarah Larney, Gabrielle Campbell, Thomas Santo, Louisa Degenhardt, Thomas Santo Jr

Theme: Epidemiology & Public Health Research Year: 2023

Background: Severe mental illness (defined here as. psychosis and bipolar disorder) is more prevalent among people who are opioid dependent, often resulting in poor clinical outcomes. Longer retention in opioid agonist treatment (OAT) is known to be associated with better health outcomes including reduced mortality. However, it is unknown whether these benefits extend to people with co-morbid severe mental illnesses. In this study, we compare OAT retention in persons with and without severe mental illness, and examine the impact of severe mental illness on all-cause mortality.

Method: The cohort comprised 18,122 people receiving OAT for the first time in New South Wales, Australia, between 2006 and 2018. The linked data resource included OAT, 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 in this population.

Results: There were 1,143 (6%) individuals diagnosed with severe mental illness and 1,585 deaths in the cohort. The risk of ceasing a first OAT episode was 15% higher for people with severe mental illness [adjusted hazard ratio (AHR) 1.15, 95% CI 1.07-1.22]. Among those with multiple OAT episodes, differences in retention between those with SMI and without were most notable between episode 1 and 2; this difference was attenuated between episodes 3-5. The risk of all-cause mortality was 85% higher in people diagnosed with severe mental illness than in those without [AHR 1.85, 95% CI 1.57-2.19].

Conclusions: Strategies to increase retention in OAT for people diagnosed with severe mental illness may reduce mortality rates in this high-risk group. Interventions to support and engage individuals in OAT are needed early after commencing OAT.

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