Terfenadine is a selective histamine H1-receptor antagonist which, in pharmacodynamic studies, is devoid of central nervous system depressant activity. In clinical studies terfenadine is well tolerated and at a dose of 60mg administered twice daily the drug provides effective relief of symptoms in patients with allergic rhinitis (seasonal and perennial), allergic dermatological conditions (particularly chronic urticaria), and other histamine-mediated disorders. Terfenadine is superior to placebo, has a more rapid onset of action than astemizole and is as effective as most other histamine H1-receptor antagonists, in relieving rhinitis symptoms. In allergic rhinitis, terfenadine relieves ocular symptoms to a greater extent (but nasal symptoms to a lesser extent) than inhaled corticosteroids. Administration of oral terfenadine with inhaled sodium cromoglycate (cromolyn sodium) or an inhaled corticosteroid appears more effective than terfenadine alone. Despite the absence of CNS depressant activity in pharmacodynamic studies, sedation is the adverse effect most frequently associated with terfenadine treatment. However, it is important to realise that the incidence of this adverse effect is similar in terfenadine and placebo recipients, and is less frequent than with traditional histamine H1-receptor antagonists. In conclusion, terfenadine is a clinically effective antihistamine which has an improved adverse effect profile compared with classic histamine H1-receptor antagonists. Like other nonsedating antihistamines, it can be considered as a first-line agent in the treatment of allergic rhinitis and chronic urticaria. With additional clinical experience, the drug could find a similar role in other disorders in which a histamine H1-receptor antagonist is indicated.