Study of Oral Atogepant When Added to OnabotulinumtoxinA (BOTOX) to Assess Adverse Events and Change in Disease Activity in Adult Participants With Chronic Migraine

Migraine is characterized by attacks of throbbing, moderate or severe headache, often associated with nausea, vomiting, and/or sensitivity to light and/or sound. The study will assess safety and tolerability of atogepant when added to BOTOX, as well as prospectively evaluate the efficacy of add-on atogepant for migraine prevention. Adverse events and change in disease activity will be monitored. Atogepant is an investigational drug being developed to prevent chronic migraine. Approximately 75 adult participants will be enrolled at approximately 30 sites in the United States. All participants will receive atogepant oral tablet once a day (QD) during the 24-week treatment period, in addition to their standard of care Botox. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and completing questionnaires.

Inclusion Criteria: * At least a 1-year history of chronic migraine (CM), with or without aura, consistent with a diagnosis according to International Classification of Headache Disorders 3rd edition (ICHD-3 2018) and with or without acute medication overuse as defined in the protocol. * Must be currently treated with BOTOX for CM: treated with \>= 2 treatment cycles in the 8 months prior to Visit 2 (Day 1) with documentation of payer authorization or written attestation of self-pay to support continued use of BOTOX. * Must have 8 to 23 (inclusive) migraine days in the electronic diary \[eDiary\] screening/baseline period (eDiary data must have been collected for at least 20 days). Exclusion Criteria: * Use of opioid-containing products for more than 4 days per month for acute treatment of headache in the 3 months prior to Screening or during the screening/baseline period. * Treatment of study target muscles using acupuncture, transcutaneous electrical nerve stimulation (TENS), cranial traction, dental splints for headache, or head and/or neck injections of anesthetics/steroids within 4 weeks prior to Screening and throughout the study. * Concurrent use of any migraine prevention treatment other than BOTOX (required concomitant medication; or topiramate \