Cetirizine, prominently marketed under the brand name Zyrtec among others, is a potent second-generation antihistamine used in the treatment of hay fever, allergies, angioedema, and urticaria.Cetirizine, prominently marketed under the brand name Zyrtec among others, is a potent second-generation antihistamine used in the treatment of hay fever, allergies, angioedema, and urticaria.