Doxylamine is an antihistamine commonly used to relieve symptoms of allergies, hay fever, and the common cold. It works by blocking the action of histamine, a substance in the body that causes allergic symptoms. Doxylamine is also often used as a short-term sleep aid due to its sedative properties. In addition to its use in over-the-counter medications, Doxylamine is sometimes combined with other ingredients to treat cold and flu symptoms. It is important to follow dosage instructions carefully, as excessive use can lead to side effects such as drowsiness, dry mouth, and dizziness.