Benadryl contains diphenhydramine, a first-generation antihistamine used for temporary relief of allergy and cold symptoms such as sneezing, watery eyes, nasal congestion, and itching.

It works by blocking histamine, a substance produced during allergic reactions. Due to its sedating effect, Benadryl may cause drowsiness and is also sometimes used as a nighttime sleep aid. Caution is advised when operating machinery or driving.