Benadryl Cough LiqGenahistMc Neil. Rx quality 100mL 100mg/5mL . Diphenhydramine 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. It is also used to prevent and treat nausea, vomiting and dizziness caused by motion sickness. Diphenhydramine can also be used to help you relax and fall asleep.This medication works by blocking a certain natural substance (histamine) that your body makes during an allergic reaction. Its .