Diphenhydramine is a prescription medication used for the management of symptoms of the common cold, hay fever and allergies. It is an antihistamine. Some of the symptoms that Diphenhydramine treats include itchy and watery eyes, runny nose, rash and sneezing. It also can help treat symptoms of motion sickness, such as dizziness and nausea. It is commonly known as the brand name Benadryl. Note that some side effects can occur from Diphenhydramine use.