Cetirizine is an oral antihistamine that works by blocking the action of histamine, a natural substance in the body involved in allergic reactions. By inhibiting histamine, cetirizine helps reduce symptoms like sneezing, itching, runny nose, and watery eyes.Cetirizine selectively blocks the histamine H1 receptor in the body, which is responsible for triggering allergic reactions. Unlike older antihistamines, cetirizine causes less drowsiness, though it may still make some individuals feel slightly tired.