Benadryl is an antihistamine. Diphenhydramine blocks the effects of the naturally occurring chemical histamine in the body. Benadryl is used to treat sneezing; runny nose; itching, watery eyes; hives; rashes; itching; and other symptoms of allergies and the common cold. Benadryl is also used to suppress coughs, to treat motion sickness, to induce sleep, and to treat mild forms of Parkinson's disease.