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.

There are critical periods of increased mortality risk at onset and cessation of opioid agonist treatment. We aim to determine whether non-fatal overdose (opioid and non-opioid) followed the same pattern as fatal opioid overdose. Comparisons were made of the first 4 weeks of treatment and treatment cessation and the remainder time off opioid agonist treatment, with the remainder time on treatment, to determine markers for intervention.

A retrospective cohort study of people with a history of opioid agonist treatment using state-wide linked New South Wales data. The incidence of non-fatal and fatal overdose hospitalisations was determined using ICD-10-AM codes, and multiple coding systems for emergency department presentations. Rates were calculated using generalized estimating equations adjusting for sex, age, calendar year, and recent health and corrective service events.

Trends were broadly similar across the four measures of overdose although at different magnitudes. The rate of drug overdose presentations to the ED was highest and almost two-fold the rate of non-opioid non-fatal overdose hospital admissions, more than three-fold the rate of opioid non-fatal overdose hospital admissions and 14 times higher than fatal opioid overdose.

Fatal overdose was lowest while in treatment, differing from the other three measures of non-fatal overdose, where the overdose rate was elevated in the first four weeks following treatment as well as the first four weeks in treatment and had a higher risk of overdose than being out of treatment longer than 28 days.

Retention on opioid agonist treatment is protective against polysubstance drug overdose. There is an excess risk of non-fatal overdose following patterns of fatal overdose at the beginning and end of treatment. The combination of non-fatal overdose with fatal overdose can be used to increase power and test delivery of treatment and other interventions to reduce overdose.