Viagra (sildenafil) is one of the most widely-known prescription drug names on the U.S. market.

Often dubbed "the little blue pill", Viagra (sildenafil) was the first phosphodiesterase 5 (PDE5) inhibitor approved to treat erectile dysfunction (ED). ED is a common sexual problem for men and its frequency increases with age.

It is estimated roughly 30 million men in the U.S. and over 100 million men worldwide suffer from ED. A large U.S. survey determined about 50 percent of men 40 to 70 years of age experience some degree of ED.