<p>Cetirizine, an antihistamine, is a histamine-1 (H1) receptor antagonist. Its effects are mediated via selective inhibition of H1 histamine receptors. The antihistaminic activity of cetirizine has been documented in a variety of animal and human models. In vivo and ex vivo animal models have shown negligible anticholinergic and antiserotonergic activity. In vitro receptor binding studies have shown no measurable affinity for other than H1 receptors.<br />
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In healthy subjects, bilateral topical ocular dosing of one drop of cetirizine ophthalmic solution 0.24% resulted in a mean cetirizine plasma Cmax of 1.7 ng/mL following a single dose and 3.1 ng/mL after twice-daily dosing for one week. The observed mean terminal half-life of cetirizine was 8.6 hours following a single dose and 8.2 hours after twice-daily dosing of Cetirizine for one week.</p>