The purpose of the study is to learn about the safety and effects of the study medicine (called ritlecitinib) for the treatment of chronic spontaneous urticaria (CSU) that is not effectively treated by antihistamines which are used to stop the body's "allergy alarm system" from overreacting. Chronic spontaneous urticaria is a disease that causes itchy hives and swellings in the deep layers of skin and fatty tissue just underneath the skin that come and go without a clear reason. This study will look at both the 50 milligrams (mg) once daily (QD) oral dose and 100 milligrams (mg) once daily (QD) oral dose and compare them to placebo (pill with no active medicine). This study is seeking participants who: * Are 18 years of age or older * Have a diagnosis of chronic spontaneous urticaria for 3 months or more that has not been controlled well with antihistamine treatment. * Do not have any other skin conditions associated with chronic itching or itching caused mainly by known triggers. * Are willing to stop all other treatments that they may be taking for chronic spontaneous urticaria other than a second-generation antihistamine (sgAH). About 150 participants will take part in this study. Participants will be chosen by chance, like drawing names out of a hat, to receive either the 50 mg dose or 100 mg dose or placebo, taken by mouth once daily for 12 weeks( Period A). Thereafter the participants on 50 mg and 100 mg will continue on their doses while the participants receiving placebo will be switched to 100mg for an additional 12 weeks (Period B). The 2 doses of ritlecitinib in this study will be compared to each other and to the placebo. This will help to see if the 50 mg and/or 100 mg dose of ritlecitinib is safe and effective. Participants will be in this study for about 8 months. During the study, participants will need to visit the study site up to 9 times. Participants will undergo various tests and procedures such as: * Physical examinations, * Hearing tests, * Blood tests, * X-ray, * ECG (electrocardiogram), * Participants will also be asked to complete questionnaires every day about their chronic spontaneous urticaria in an electronic diary.
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Change from baseline in Urticaria Activity Score 7 (UAS7) at Week 12
Timeframe: Week 12
Incidence of Treatment Emergent Adverse Events, Serious Adverse Events, and Adverse Events leading to discontinuation
Timeframe: Week 12
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