The nonsedating histamine H1 receptor antagonist fexofenadine is the active metabolite of terfenadine. It reduced the allergic response in animal models of allergy and did not prolong the QT interval (QTc) in dogs or rabbits at plasma concentrations many times higher than those seen after administration of therapeutic dosages. Similarly, relative to placebo, fexofenadine did not affect mean QTc in patients given dosages of up to 480 mg/day for 2 weeks or in volunteers who received up to 800 mg/day for 6 days or 240 mg/day for 12 months. In a double-blind clinical trial, oral fexofenadine 120 or 180mg once daily controlled symptoms in patients with seasonal allergic rhinitis as effectively as cetirizine. Other double-blind clinical trials showed that fexofenadine 40 to 240mg twice daily was significantly more effective than placebo. Fexofenadine 180 or 240mg once daily was significantly more effective than placebo in patients with chronic idiopathic urticaria. The drug was well tolerated in these clinical trials, with an adverse event profile similar to that seen with placebo. The most common adverse events were headache, throat irritation, viral infection, nausea, dysmenorrhoea, drowsiness, dyspepsia and fatigue.