While the manufacturer of Xeljanz, Pfizer, does not warn of an interaction between alcohol and Xeljanz, it is generally advised not to drink excessive amounts of alcohol while taking Xeljanz. This is because both alcohol and Xeljanz can affect the liver, and the risk may be higher if you take other immunosuppressants or medications that also affect the liver, such as methotrexate, or if you already have liver disease.
