What is Diphenhydramine? Diphenhydramine (also known as DPH, and by its trade name Benadryl, among many others) is a first-generation antihistamine used to treat a number of conditions including allergic symptoms, itchiness, insomnia, motion sickness, Parkinson’s disease and extra-pyramidal symptoms. Diphenhydramine may also be used for purposes not listed in this medication guide.