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. 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. 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 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. 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. Key Takeaways Benadryl can cause drowsiness and should not be used when driving. Taking large doses of Benadryl can lead to serious health issues like cardiac arrest and death. People with glaucoma should talk to a doctor before using Benadryl because it can cause vision problems. 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. Find out how diphenhydramine treats symptoms of allergies and short-term insomnia, and how to take it. NHS medicines information on diphenhydramine – what it's used for, side effects, dosage and who can take or use it. Diphenhydramine, 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.