DPH is an abbreviation for the chemical "Diphenhydramine" (dye-fen-hye'-dra-meen). It is the active ingredient in many over the counter sleep aids including Unisom, Nytol, Tylenol PM (combined with acetaminophen) and in many generic store brand remedies. It is also the active ingredient in Benadryl and is very closely related to the chemical Dimenhydrinate which is the active ingredient in Dramamine.
Diphenhydramine is classified as an antihistamine, antiemetic, sedative, and hypnotic. It is also an anticholinergic deliriant in high doses ( 150-800 mg's). Some people consider it recreational in these high doses but the uncomfortability of the high keeps it's abuse potential to a minimum. DPH is also used recreationally in lower doses (25-100mg's) in combination with opiates and benzodiazepines for it's potentiation abilities.
Last night I railed an OC 80 and took 75 mg's of DPH. Man I was noddin' hard!