Time-Varying Vulnerability to Non-Fatal Overdose: A Self-Controlled Case Series

IntroductionCohort studies have suggested that there are periods of time, including the two weeks following release from prison and the transition on and off opioid agonist therapy (OAT), where the risk of overdose is heightened. However, this research has focused on fatal overdose and may be subject to confounding. Objectives and ApproachThis study aimed to examine the association between time-varying risk factors – release from incarceration, discharge from hospital and emergency department, and use of prescribed OAT, opioids, benzodiazepines and antipsychotics – and non-fatal overdose. People in a 20% random sample of residents in BC, Canada who experienced a non-fatal overdose in 2015-2017 were identified through hospital admissions, physician and emergency department visits, and poison centre and ambulance calls. Risk periods associated with exposure to each time varying risk factor were created using linked administrative data. Using a self-controlled case series design, conditional Poisson regression was used to estimate the incidence rate ratio of non-fatal overdose during the risk periods compared to at other times. Results4149 people experienced a non-fatal overdose during follow-up. People were at increased risk of overdose on the day of admission to prison (adjusted incidence rate ratio (AIRR) 2.8, 95% confidence interval (95%CI) 1.5-5.0), in the two weeks after release from prison (AIRR 2.9, 95%CI 2.4-3.6) and after hospital discharge (AIRR 1.3, 95%CI 1.1-1.6), and during prescription opioid (AIRR 1.3, 95%CI 1.0-1.6) and benzodiazepine (AIRR 1.7, 95%CI 1.3-2.1) use. People were at lower risk of non-fatal overdose during OAT use (AIRR 0.4, 95%CI 0.3-0.5) and while in prison (AIRR 0.1, 95%CI 0.1-0.2). Conclusion / ImplicationsThere are acute, transient periods where a person’s risk of overdose is heightened. These include release from incarceration, discharge from hospital, and while taking prescription opioids and benzodiazepines. These periods of increased risk should be targeted for overdose prevention efforts.