Benadryl starts working within 30 minutes after taking it orally, with peak effects reached in 1-2 hours. The antihistamine effects last 4-6 hours for most people. Injectable Benadryl works even faster than oral tablets, providing quicker relief for severe allergic reactions. 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. 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. Find BENADRYL® dosage information for adults, children, and toddlers and learn how much you and your family should take with our allergy medicine product-dosing guide. Benadryl® (diphenhydramine) is a liquid medication that treats the symptoms of allergies and allergic reactions. It may also prevent and treat motion sickness or the symptoms of Parkinson’s disease. Benadryl (diphenhydramine) is an antihistamine used to treat allergies, hives, insomnia, motion sickness, and mild cases of Parkinsonism. Benadryl is available in generic form and over-the-counter (OTC). Do you need help relieving allergies or a pesky itch? Check out the many allergy and itch relief products BENADRYL® offers which may help provide relief. Benadryl is an antihistamine and is used to treat sneezing, runny nose, itching and other allergies. Learn about side effects, interactions and indications. Benadryl is a brand of various antihistamine medications used to treat allergies, whose content varies in different countries, but which includes some combination of diphenhydramine, acrivastine, or cetirizine. Benadryl (diphenhydramine) is a brand-name over-the-counter drug that’s used for allergies and itchy skin. Learn about side effects, price, and more.