Atogepant is an orally administered, small-molecule, calcitonin gene-related peptide (CGRP) receptor antagonist under investigation for treatment of migraine. We aimed to examine a range of oral doses for safety, tolerability, and efficacy for the preventive treatment of migraine.In this double-blind, phase 2b/3 trial, adults (aged 18-75 years), with a history (≥1 year) of migraine and 4-14 migraine days per month, were randomly assigned 2:1:2:2:1:1 (by means of a sequence generated by the statistical programming department of the sponsor, and operationalised through an automated interactive web-based response system) to receive placebo or atogepant 10 mg once daily, 30 mg once daily, 60 mg once daily, 30 mg twice daily, or 60 mg twice daily, in matching capsules. Participants, site personnel, and all study sponsor personnel were masked to treatment allocations. The study was done in 78 academic and private practice settings in the USA. The primary outcome was change from baseline in monthly migraine days across 12 weeks of treatment using a modified intention-to-treat approach. The overall type I error rate for multiple comparisons across active treatment doses was controlled at the 0·05 level by means of a graphic approach. The main outcomes to assess safety and tolerability were adverse event recordings. The trial is registered with ClinicalTrials.gov, NCT02848326 and is completed.Between Sept 6, 2016, and April 23, 2018, of 1772 individuals screened, 834 were randomly assigned and 825 received one dose or more of study medication: 186 received placebo, 93 atogepant 10 mg once daily, 183 atogepant 30 mg once daily, 186 atogepant 60 mg once daily, 86 atogepant 30 mg twice daily, and 91 atogepant 60 mg twice daily. Overall, 714 (87%) of 825 participants were female, 628 (76%) were white, median migraine duration was 17·5 years (IQR 10·0-28·0), and 232 (28%) had previously used preventive treatment. The primary efficacy analysis included 795 patients: 178 received placebo, 92 atogepant 10 mg once daily, 182 atogepant 30 mg once daily, 177 atogepant 60 mg once daily, 79 atogepant 30 mg twice daily, and 87 atogepant 60 mg twice daily. Across the 12-week treatment period, all five atogepant groups showed significant least-squares mean (SE) change from baseline in mean monthly migraine days versus placebo: atogepant 10 mg once daily -4·0 (0·3; p=0·024), 30 mg once daily -3·8 (0·2; p=0·039), 60 mg once daily -3·6 (0·2; p=0·039), 30 mg twice daily -4·2 (0·4; p=0·0034), and 60 mg twice daily -4·1 (0·3; p=0·0031); placebo -2·9 (0·2). The most common treatment-emergent adverse events (TEAEs) across all groups were nausea (range 5% [5/93] for 10 mg once daily to 12% [22/186] for 60 mg once daily vs 5% [9/186] for placebo) and fatigue (1% [1/93] for 10 mg once daily to 10% [9/91] for 60 mg twice daily vs 3% [6/186] for placebo). Treatment-related TEAE frequency ranged from 18% (17/93) for 10 mg once daily to 26% (24/91) for 60 mg twice daily, versus 16% (30/186) for placebo. Seven participants reported a total of eight serious TEAEs (two participants each in the placebo, 30 mg once-daily, and 60 mg once-daily groups, and one participant in the 10 mg once-daily group). TEAEs leading to discontinuation were reported in 33 (5%) of 639 atogepant participants and 5 (3%) of 186 of those randomised to placebo. All serious TEAEs were unrelated to treatment.All doses of oral atogepant were associated with a significant decrease in monthly migraine days over 12 weeks compared with placebo. Atogepant was safe and well tolerated over 12 weeks, supporting its phase 3 development for the preventive treatment of migraine.Allergan (before its acquisition by AbbVie).