Diphenhydramine is a prescription medication used to treat the symptoms of an allergic reaction, insomnia, cough, motion sickness, and parkinsonism. Learn about side effects, dosages, drug interactions, warnings, and more. A first-generation antihistamine approved in 1946, diphenhydramine is widely used for allergies, sleep aid and cold symptoms. Diphenhydramine, or Diphenylhydramine, sold under the brand name Benadryl among others, is an antihistamine and sedative. Although generally considered sedating, diphenhydramine can cause paradoxical central nervous system stimulation in some individuals, particularly at higher doses. Benadryl (diphenhydramine) is a first‑generation antihistamine that often makes you sleepy for around 8 hours. It can also cause confusion, falls, or ‘paradoxical’ agitation, so it’s not recommended as a regular sleep aid, especially in children and older adults. Diphenhydramine is an antihistamine medication that treats the symptoms of allergies and allergic reactions. It also prevents and treats motion sickness or symptoms of Parkinson’s disease. NHS medicines information on diphenhydramine – what it's used for, side effects, dosage and who can take or use it. Diphenhydramine is used to treat sneezing, runny nose, watery eyes, hives, skin rash, itching, and other cold or allergy symptoms. Diphenhydramine is also used to treat motion sickness, to induce sleep, and to treat certain symptoms of Parkinson's disease. Learn about diphenhydramine usage and dosing. Read the latest news and reviews about the drug as well as potential side effects and popular alternatives. Overview: Diphenhydramine is an antihistamine that is used to help treat certain allergy or common cold symptoms, allergic reactions, trouble sleeping, motion sickness, or certain movement. Diphenhydramine is in a class of medications called antihistamines. It works by blocking the action of histamine, a substance in the body that causes allergic symptoms.