Erectile dysfunction (ED), also called impotence, is characterized by a man's inability to have an erection hard enough to have sex or to keep an erection long enough to finish having sex.

ED affects over 15 million men in the U.S. The normal physiology of erection involves a complex series of events. The penis contains two cylinders of sponge-like tissue that run along it's shaft, parallel to the urethra. When sexual excitation occurs, blood flow to this tissue normally increases. As the tissue...