Benadryl, known by its medical name as diphenhydramine, is an antihistamine used to treat allergic reactions and seasonal allergies. Ingredients vary depending on the type of Benadryl medication being used, but all Benadryl medications contain diphenhydramine. A fatal dose of diphenhydramine is about 20 to 40 milligrams per kilogram (mg/kg). Learn more about the fatal … Continue reading What Is A Lethal Dose Of Benadryl (Diphenhydramine)?