Benadryl Active ingredient: diphenhydramine $1.73 for pill Benadryl is an antihistamine. Diphenhydramine blocks the effects of the naturally occurring chemical histamine in the body. It is used to treat sneezing; runny nose; itching, watery eyes; hives; rashes; and other symptoms of allergies and the common cold. Sominex Active ingredient: acetaminophen, diphenhydramine $0.57 for pill Sominex is used to induce sleep, to suppress coughs, to treat motion sickness, and to treat mild forms of Parkinson's disease.