What are the differences between cetirizine and singulair (montelukast)?

Different classes: Cetirizine is a new generation antihistamine - it works by blocking histamine so is helpful for allergies and hives. Singulair (montelukast) is a leukotriene receptor antagonist and blocks leukotrienes. It does not block histamine but is approved for treating allergies and asthma. Since they are different, a person can take both together if needed.