How to Recognize the Signs of Benadryl Abuse Benadryl is a common OTC medicine for allergy relief and as a sleep aid. However, it can have severe side effects when combined with other medications or abuse. Article Contents What is Benadryl? Benadryl is a very common antihistamine that helps relieve symptoms of allergies and hay fever such as itching, rash, and watery eyes. It is typically an over-the-counter, or OTC, medicine. However, in some instances, it is considered a prescription medication when prescribed by