Benadryl is the brand name for diphenhydramine, an antihistamine used to alleviate symptoms of sneezing, runny nose, cough and watery eyes that accompany the common cold, allergies and hay fever. It is combined with pain-relieving drugs in popular over-the-counter sleep aids, according to WebMD.
Benadryl works by blocking histamines that the body naturally makes in the throes of an allergic reaction, explains WebMD. The drying effect of Benadryl comes from its ability to block acetylcholine in the body, making it useful in alleviating a runny nose or watery eyes. Benadryl comes in both tablet and capsule form and is also available as a liquid.