Used as an antihistamine to relieve symptoms of allergies, hay fever, and the common cold such as sneezing, runny nose, and itching. It works by blocking histamine, a substance in the body that causes allergic reactions. Often found in over-the-counter cold and allergy medications, especially in liquid or tablet form. May cause drowsiness, so it is sometimes included in nighttime symptom relief products. Also used in some pediatric formulations due to its fast-acting nature.