Background: The goal of the study was to assess the safety and tolerability of atogepant, an oral, calcitonin gene-related peptide receptor antagonist in development for migraine preventive treatment, once daily over 1 year. Methods: Multicenter, open-label trial (NCT03700320). Adults with migraine were randomized 5:2 to atogepant or oral standard-of-care (SOC) migraine prevention. Results: 744 randomized participants (n=546 atogepant), 739 safety population participants (n=543 atogepant). Adverse events (AEs) were reported by 67.0% of atogepant participants; 18.0% had AEs considered related to atogepant. AEs reported by ≥5% of atogepant-treated participants were upper respiratory tract infection (10.3%), constipation (7.2%), nausea (6.3%), and urinary tract infection (5.2%). 4.4% of atogepant participants reported serious AEs that included various, common medical conditions; no event occurred in ≥1 participant and none were atogepant-related. Two deaths were reported in atogepant-treated participants (homicide victim; toxic shock syndrome); both were considered not treatment-related. 5.7% of atogepant participants discontinued due to AEs. Alanine aminotransferase/aspartate aminotransferase levels ≥3X upper limit of normal were reported for 2.4% of atogepant participants (n=13/531) and 3.2% of SOC participants (n=6/190). No cases of potential Hy’s Law were reported. Conclusions: Once-daily use of atogepant for preventive treatment of migraine over 1 year was safe and well-tolerated with no safety concerns identified.