Diphenhydramine is an antihistamine mainly used to treat allergies.[6] It is also used for insomnia, symptoms of the common cold, tremor in parkinsonism, and nausea.[6] It is used by mouth, injection into a vein, and injection into a muscle.[6] Maximal effect is typically around two hours after a dose, and effects can last for up to seven hours.[6]

Common side effects include sleepiness, poor coordination, and an upset stomach.[6] Its use is not recommended in babies.[6] There is no clear risk of harm when used during pregnancy; however, use during breastfeeding is not recommended.[7] It is a first generation H1-antihistamine and works by blocking certain effects of histamine.[6] Diphenhydramine is also an anticholinergic.[8]

Diphenhydramine was first made by George Rieveschl and came into commercial use in 1946.[9][10] It is available as a generic medication.[6] The wholesale price in the developing world as of 2014 is about US$0.01 per dose.[11] In the United States, it costs less than US$25 for a typical month’s supply.[12] It is sold under the trade name Benadryl, among others.[6]