We aim to characterize the incidence and risk factors for opioid-related and all-cause mortality in the year after an emergency department (ED) visit for nonfatal opioid poisoning by conducting a population-based study.We used linked health care databases in Ontario, Canada, to identify individuals who attended an ED for nonfatal opioid poisoning between January 1, 2015, and December 31, 2016. Using Cox proportional hazards regression, we examined predictors of mortality in the year after discharge (ED or hospital, if admitted).In this cohort (n=6,140), 327 individuals (5.3%) died of any cause and 118 (1.9%) died of opioid-related causes within 1 year. Adjusting for other covariates, we found that health service use in the first week was not protective for opioid-related death (hazard ratio [HR] 0.70; 95% confidence interval [CI] 0.47 to 1.06) or all-cause mortality (HR 0.98; 95% CI 0.78 to 1.24). In exploring other covariates, predictors of opioid-related mortality included male sex (HR 1.98; 95% CI 1.32 to 2.97) and using opioid agonist therapy (HR 1.79; 95% CI 1.15 to 2.80) or benzodiazepine (HR 1.54; 95% CI 1.02 to 2.31) in the 12 months before the index event. Assessment by a family physician in the previous 12 months was associated with a lower risk of opioid-related and all-cause mortality (HR 0.58, 95% CI 0.39 to 0.86; and HR 0.63, 95% CI 0.49 to 0.82, respectively).We identified predictors of opioid-related and all-cause mortality after ED presentation for opioid poisoning. Several predictors of mortality may facilitate targeted interventions.