Cetirizine is a non-sedating antihistamine that works by blocking histamine (H-1) receptors on cells. It is similar to the other second generation antihistamines loratadine ( Claritin ), fexofenadine ( Allegra ) and azelastine ( Astelin ). Histamine is a chemical that is responsible for many of the signs and symptoms of allergic reactions, for example, swelling of the lining of the nose, sneezing , and itchy eyes. Histamine is released from histamine-storing cells (mast cells) and then attaches to other cells that have receptors for histamine. The attachment of the histamine to the receptors causes the cells to be "activated," releasing other chemicals that produce the effects that we associate with allergy , for example, sneezing . Certirizine blocks one type of receptor for histamine (the H1 receptor) and thus prevents activation of H1 receptor-containing cells by histamine. Unlike the first generation antihistamines, cetirizine and other second-generation antihistamines do not readily enter the brain from the blood, and, therefore, they cause less drowsiness. Cetirizine may cause more drowsiness than other second generation antihistamines. The FDA approved cetirizine in September 1996.