BACKGROUND: The efficacy of opioids for acute pain relief in the emergency department (ED) is well recognized, but treatment with opioids is associated with adverse events ranging from minor discomforts to life-threatening events.OBJECTIVE: To assess the impact of age, sex and route of administration on the incidence of adverse events due to opioid administration in the ED.METHODS: Real-time archived data were analyzed retrospectively in a tertiary care urban hospital. All consecutive patients (≥16 years of age) who were assigned to an ED bed and received an opioid between March 2008 and December 2012 were included. Adverse events were defined as: nausea/vomiting (minor); systolic blood pressure (SBP) < 90 mmHg, oxygen saturation (Sat) < 92% and respiration rate < 10 breaths/min (major) within 2 h of the first opioid doses.RESULTS: In the study period, 31,742 patients were treated with opioids. The mean (± SD) age was 55.8± 20.5 years, and 53% were female. The overall incidence of adverse events was 12.0% (95% CI 11.6% to 12.4%): 5.9% (95% CI 5.6% to 6.2%) experienced nausea/vomiting, 2.4% (95% CI 2.2% to 2.6%) SBP < 90 mmHg, 4.7% (95% CI 4.5% to 4.9%) Sat that dropped to < 92% and 0.09% respiration rate < 10 breaths/min. After controlling for confounding factors, these adverse events were associated with: female sex (more nausea/vomiting, more SBP < 90 mmHg, less Sat < 92%); age ≥65 years (less nausea/vomiting, more SBP < 90 mmHg, more Sat < 92%); and route of administration (intravenous > subcutaneous > oral).CONCLUSIONS: The incidence of adverse events associated with opioid administration in the ED is generally low and is associated with age, sex and route of administration.