Objective: Assess the long-term efficacy and safety of erenumab in patients with chronic migraine with acute medication overuse. Background: Overuse of acute medication in patients with chronic migraine has been linked to greater pain intensity and disability and may diminish the effectiveness of preventive therapies. Methods: This 52-week open-label extension study followed a 12-week double-blind placebo-controlled study in which patients with chronic migraine were randomized 3:2:2 to placebo or once-monthly erenumab 70 mg or 140 mg. Patients were stratified by region and medication overuse status. Patients received erenumab 70 mg or 140 mg throughout or switched from erenumab 70 to 140 mg (based on protocol amendment to augment safety data at higher dose). Efficacy was assessed in patients with and without medication overuse at parent study baseline. Results: Of 609 patients enrolled in the extension study, 252/609 (41.4%) met the criteria for medication overuse at parent study baseline. At Week 52, the mean change in monthly migraine days from parent study baseline was −9.3 (95% confidence interval: −10.4, −8.1 days) in the medication overuse subgroup versus −9.3 (−10.1, −8.5 days) in the non-medication overuse subgroup (combined erenumab doses); proportion of patients achieving ≥50% reduction in monthly migraine days at Week 52 was 55.9% (90/161; 48.2%, 63.3%) versus 61.3% (136/222; 54.7%, 67.4%), respectively. Among baseline users of acute migraine-specific medication, the mean change in monthly migraine-specific medication days at Week 52 was −7.4 (−8.3, −6.4 days) in the medication overuse subgroup versus −5.4 (−6.1, −4.7 days) in the non–medication overuse subgroup. Most patients (197/298; 66.1%) in the medication overuse subgroup transitioned to non-overuse status by Week 52. Erenumab 140 mg was associated with numerically greater efficacy than erenumab 70 mg across all endpoints. No new safety signals were identified. Conclusion: Long-term erenumab treatment demonstrated sustained efficacy and safety in patients with chronic migraine with and without acute medication overuse.