Cetirizine D4 is a deuterium labeled Cetirizine. Cetirizine, a second-generation antihistamine and the carboxylated metabolite of hydroxyzine, is a specific, orally active and long-acting histamine H1-receptor antagonist. Cetirizine marks antiallergic properties and inhibits eosinophil chemotaxis during the allergic response^[1][2][3].