Cetirizine, the carboxylated metabolite of hydroxyzine, is a specific and long-acting histamine H₁-receptor antagonist. It has marked antiallergic properties and inhibits eosinophil chemotaxis during the allergic response. Clinical trial results indicate that cetirizine is an effective and well tolerated treatment for seasonal/perennial allergic rhinitis and chronic idiopathic urticaria in adults, and for seasonal/perennial allergic rhinitis in children. Cetirizine 10 mg/day appears to be as effective as conventional dosages of other established antihistamines such as astemizole, hydroxyzine, ketotifen, loratadine or terfenadine in relieving symptoms of these disorders, and is associated with a significantly lower incidence of sedation than hydroxyzine. However, when sedation was subjectively assessed, cetirizine appeared to be more sedating than placebo, loratadine or terfenadine in some clinical trials. This difference was not observed in several other double-blind studies. In contrast, when assessed objectively in pharmacodynamic comparisons, cetirizine was rarely more sedating than placebo or other second generation histamine H₁-receptor antagonists. Cetirizine may also have a role in the treatment of certain forms of physical urticaria, atopic dermatitis and reactions to mosquito bites. In addition, it is being studied for the treatment of allergic asthma in adults and children. The pharmacokinetic profile and predominantly renal excretion of cetirizine suggest that this agent may have a reduced potential for adverse drug interactions involving hepatic enzyme systems compared with other histamine H₁-receptor antagonists which are extensively metabolised. Thus, cetirizine, with its rapid onset and long duration of action, appears to provide a useful alternative to the antihistamine agents in clinical use.

Cetirizine is a specific histamine H₁-receptor antagonist which has greater antihistaminic activity than clemastine, hydroxyzine, mepyramine and terfenadine in animal models. In atopic and nonatopic volunteers, cetirizine 10mg, administered as a single dose or daily for 2 to 28 days, significantly suppressed histamine-, allergen- and antigen-induced weal and flare response compared with placebo. Following histamine challenge, this effect peaked 4 to 8 hours after administration of cetirizine and lasted up to 24 hours after a single dose. Against histamine-induced weal and flare, cetirizine 10mg had similar activity to diphenhydramine 50mg or hydroxyzine 25mg, and was at least as effective as standard doses of astemizole, azelastine, brompheniramine, chlorphenamine (chlorpheniramine), clemastine, cyproheptadine, ebastine, ketotifen, loratadine, mequitazine, oxatomide or terfenadine. In addition, it had a significantly more rapid onset of action than astemizole 10mg against histamine-induced weal and flare. In antiallergic assessments of weal and flare response, cetirizine 10mg was as active as astemizole 10mg, ketotifen 1mg or terfenadine 120mg, and was significantly more active than chlorphenamine 8mg or loratadine 10mg. A 20mg dose of cetirizine was also superior to clemastine 2mg in antiallergic studies.

Cetirizine 5 to 20mg provided dose-dependent protection against inhaled histamine-induced bronchospasm in patients with asthma, and protected some patients from allergen-induced bron-chospasm. The immediate and late responses to allergen challenge in patients with allergic rhinitis were also attenuated by cetirizine 10 mg/day. The effect of cetirizine 10mg on histamine-induced bronchial response was significantly greater than that of astemizole …