Pharmacodynamics: Cetirizine is a potent antihistamine with a low potential for drowsiness at normal therapeutic doses which has additional antiallergic properties. It is a selective H1-antagonist with negligible effects on other receptors and so, is virtually free from anticholinergic and anti-serotonin effects. Cetirizine inhibits the histamine-mediated early phase of the allergic reaction and also reduces the migration of certain inflammatory cells abd the release of certain mediators associated with the late allergic response.
Pharmacokinetics: Peak blood levels of the order of 0.3 mcg/mL are reached between 30 and 60 min after the oral administration of a 10-mg dose of cetirizine. The terminal half-life is approximately 10 hrs in adult and 6 hrs in children 6-12 yrs. This is consistent with the urinary excretion half-life of the drug. The cumulative urinary excretion represents about two-thirds of the administered dose for both adults and children. The apparent plasma clearance in children is higher than that measured in adults. A high proportion of cetirizine is bound to human plasma proteins.