Cetirizine is a widely used, second-generation antihistamine. It is a major metabolite of the first-generation drug hydroxyzine but has been chemically modified to be significantly less able to cross the blood-brain barrier. It is highly effective for the symptomatic treatment of seasonal and perennial allergic rhinitis (hay fever), as well as chronic idiopathic urticaria (hives), providing long-lasting relief from itching, sneezing, and runny nose.