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Introduction: After treatment with naltrexone extended-release injectable suspension (XR-NTX), a µ-opioid receptor antagonist, opioid tolerance is reduced from pretreatment baseline. Patients may be vulnerable to opioid overdose if they attempt to override the blockade during treatment, at the end of a dosing interval, after missing a dose, or after discontinuing treatment. Objective: We analyzed postmarketing data to characterize reporting rates of opioid overdose during treatment with and after discontinuation of XR-NTX. Methods: Postmarketing adverse event reports within the XR-NTX safety database, received 2006–2018, for patients treated with XR-NTX for any indication were reviewed for opioid overdose cases. Assessable cases were categorized by timing of the event from the last dose of XR-NTX (latency): ≤28 days (on treatment), 29–56 days, and >56 days from last dose of XR-NTX. Within each latency group, cases were further classified as serious and, of those, cases that had a fatal outcome. Results: During the 12-year period, an estimated 495,602 patients received XR-NTX. Opioid overdose was reported in 161 cases; of these, 66 contained sufficient information to determine latency. Reporting rates of opioid overdose per 10,000 patients treated were similar among latency groups: 0.54 for ≤28 days (0.24 fatal), 0.34 for 29–56 days (0.16 fatal), and 0.44 for >56 days (0.40 fatal) from the last dose of XR-NTX. Conclusions: Over the 12-year period, the reporting rates of opioid overdose were similar during treatment with or after discontinuation of XR-NTX and <10/10,000 patients exposed. Our findings are limited by the nature of spontaneously reported safety data. [ABSTRACT FROM AUTHOR]