At one time or another nearly everyone has taken diphenhydramine (dye-fen-HYE-dra-meen), an antihistamine that can have a sedating effect on the central nervous system. Millions of people take it for dust, pollen, and other allergies; others for nausea, vomiting, or vertigo; some for Parkinson’s disease. And untold millions use it as a sleep aid. Best known under the brand