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. 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. Allergy (Diphenhydramine HCl) is an antihistamine that is used to treat sneezing, runny nose, watery eyes, hives, skin rash, itching, and other cold or allergy symptoms. Allergy (Diphenhydramine HCl) is also used to treat motion sickness, to induce sleep, and to treat certain symptoms of Parkinson's disease. The active ingredient in BENADRYL ® is an antihistamine called diphenhydramine hydrochloride or diphenhydramine HCl. Antihistamines are used for relief from symptoms related to hay fever, upper respiratory allergy, or cold symptoms. Learn about diphenhydramine usage and dosing. Read the latest news and reviews about the drug as well as potential side effects and popular alternatives. 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. How does diphenhydramine work (mechanism of action)? Diphenhydramine is an antihistamine. It works by blocking the effect of a substance in your body called histamine. 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. Diphenhydramine hydrochloride is an antihistamine used to relieve symptoms of allergy, hay fever, and the common cold. These symptoms include rash, itching, watery eyes, itchy eyes/nose/throat, cough, runny nose, and sneezing. 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.