A Study to Learn About the Study Medicine Called Rimegepant in Adults When Used for the Prevention of Chronic Migraine

Inclusion Criteria:

* Male or female participants ≥18 years of age.
* Diagnosis of chronic migraine, with or without aura, according to International Classification of Headache Disorders, 3rd edition (ICHD-3) criteria.
* History of migraine for at least 1 year before screening.
* Migraine onset before 50 years of age and chronic migraine onset before 65 years of age.
* Migraine attacks lasting 4 to 72 hours when untreated or unsuccessfully treated.
* During the 3 months prior to screening, an average of:
* ≥15 monthly headache days (MHDs),
* including ≥8 monthly migraine days (MMDs),
* and ≥1 non-headache day per month.
* Completion of the Observation Phase and fulfillment of all protocol-defined eligibility requirements prior to randomization.
* Willing and able to comply with study procedures, including completion of the electronic diary.

Exclusion Criteria:

* Prior use of rimegepant for migraine prevention.
* History of treatment failure of:

  * 2 gepants, or
  * 2 CGRP-targeting biologics, or
  * 1 gepant and ≥1 CGRP-targeting biologic.
* Failure of \>4 migraine preventive medications, including at least 2 with different mechanisms of action.
* Any medical or psychiatric condition, or clinically significant laboratory abnormality, that in the investigator's judgment may increase risk to the participant or interfere with study participation or interpretation of results. This includes, but is not limited to, uncontrolled hypertension or a history of Raynaud's phenomenon.
* Pregnant, breastfeeding, planning to become pregnant, or planning to initiate breastfeeding during the study.
* Positive amphetamine screening result unless attributable to prescribed stimulant therapy that has been stable for at least 3 months prior to Visit 1 and is expected to remain stable throughout study participation.
* Use of investigational products or participation in another clinical study within protocol-defined exclusion windows.
* Any other condition that, in the opinion of the investigator or sponsor, would make study participation unsafe or compromise study conduct.