Background: Fremanezumab, a fully humanized monoclonal antibody (IgG2Δa) selectively targets the calcitonin gene-related peptide and has proven efficacy for the preventive treatment of migraine. In this study, we evaluated the long-term efficacy, safety, and tolerability of monthly and quarterly fremanezumab. Methods: Episodic migraine and chronic migraine patients completing the 12-week double-blind period of the FOCUS trial entered the 12-week open-label extension and received 3 monthly doses of fremanezumab (225 mg). Changes from baseline in monthly migraine days, monthly headache days of at least moderate severity, days of acute headache medication use, days with photophobia/phonophobia, days with nausea or vomiting, disability scores, and proportion of patients achieving a ≥50% or ≥75% reduction in monthly migraine days were evaluated. Results: Of the 807 patients who completed the 12-week double-blind treatment period and entered the open-label extension, 772 patients completed the study. In the placebo, quarterly fremanezumab, and monthly fremanezumab dosing regimens, respectively, patients had fewer average monthly migraine days (mean [standard deviation] change from baseline: − 4.7 [5.4]; − 5.1 [4.7]; − 5.5 [5.0]), monthly headache days of at least moderate severity (− 4.5 [5.0]; − 4.8 [4.5]; − 5.2 [4.9]), days per month of acute headache medication use (− 4.3 [5.2]; − 4.9 [4.6]; − 4.8 [4.9]), days with photophobia/phonophobia (− 3.1 [5.3]; − 3.4 [5.3]; − 4.0 [5.2]), and days with nausea or vomiting (− 2.3 [4.6]; − 3.1 [4.5]; − 3.0 [4.4]). During the 12-week open-label extension, 38%, 45%, and 46% of patients, respectively, achieved a ≥50% reduction and 16%, 15%, and 20%, respectively, achieved a ≥75% reduction in monthly migraine days. Disability scores were substantially improved in all 3 treatment groups. There were low rates of adverse events leading to discontinuation (<1%). Conclusion: Fremanezumab demonstrated sustained efficacy up to 6 months and was well tolerated in patients with episodic migraine or chronic migraine and documented inadequate response to multiple migraine preventive medication classes. Trial registration: ClinicalTrials.gov NCT03308968 (FOCUS).