The ingredient in Benadryl that causes drowsiness is diphenhydramine. Commonly used as an antihistamine to treat allergies, diphenhydramine is also used as a sleep aid. Diphenhydramine is also classified as a CNS depressant, sedative and antiemetic.
Following oral administration, dependent on dosage, drowsiness and other common side effects normally last between 4 to 6 hours. The side effects of diphenhydramine, such as drowsiness, are worsened by the use of alcohol, sedatives and tranquilizers, as well as MAOIs, other CNS depressants and hypnotics. These effects are more pronounced in the elderly.
Some other notable side effects of diphenhydramine include agitation, confusion, fatigue, irritability, paresthesia, blurred vision and depression. In overdose, symptoms may include excitement, ataxia, tremor, sinus tachycardia, fever, hallucination, athetosis, convulsions, seizures, hypotension, deep coma, cardiorespiratory collapse and death.