The impact of opioid agonist treatment on fatal and non-fatal drug overdose among people with a history of opioid dependence in NSW, Australia, 2001-2018: Findings from the OATS retrospective linkage study

Background: There are critical periods of mortality risk at onset and cessation of opioid agonist treatment. We aim to determine whether non-fatal overdose followed the same pattern as fatal overdose, comparing the first 4 weeks of treatment and treatment cessation and the remainder time off treatment, with the remainder treatment time, to determine intervention markers.Methods: Retrospective cohort study of people with a history of opioid agonist treatment using linked New South Wales data. The incidence of non-fatal overdose hospitalization; emergency department presentation; and fatal overdose from national death records were compared. Rates were calculated using generalized estimating equations adjusting for demographics, year, and recent health and incarceration events. Results: The remainder time in OAT had the lowest incidence of overdose for all outcomes and is the reference level for the adjusted incident rate ratios (aIRR). Fatal overdose was lowest in treatment and highest in the first four weeks out of treatment, aIRR of 12.83 (95% CI 10.0-16.4). Whereas the highest overdose rate for non-fatal opioid overdose was in the first four weeks in treatment, aIRR of 3.11 (95% CI 2.19-4.42).Conclusions: Retention on opioid agonist treatment is protective against drug related overdose. There is elevated risk of non-fatal overdose at treatment initiation that is not evident for fatal overdose, but the first month of treatment cessation is a critical period for both non-fatal and fatal overdose. These findings emphasize the importance of treatment retention and interventions for polysubstance overdose at cessation.