Since Viagra® (sildenafil citrate)—the first oral drug to treat erectile dysfunction—went on sale ten years ago, more than 27 million men in 120 countries have been prescribed it for impotence. Initially the drug was designed to treat high blood pressure, but in clinical trials this use proved disappointing. One side effect, shyly reported by the healthy volunteers, was that the drug produced super-charged erections.
It was known that sexual arousal messages from the brain spark the production of cyclic guanosine monophosphate (cyclic GMP), a chemical that relaxes the pelvic muscles and allows the penis to become engorged with eight times its normal supply of blood. Sildenafil suppresses an enzyme (phosphodiesterase type 5), whose normal role is to break down cyclic GMP and cause the erection to subside.
Pharmaceutical company Pfizer conducted twenty-one randomized, placebo-controlled clinical trials involving more than 3,700 participants aged from nineteen to eighty-seven suffering from varying degrees of impotence. Results showed that Viagra'8 restored sexual function in seven out of ten men. Fortunately, the drug worked only when the men were sexually aroused, and not at other times also.
The drug was approved by the U.S. Food and Drug Administration in 1998. Previous treatments for erectile dysfunction included injections into the penis, urethral suppositories, surgery, and vacuum devices. Within fourteen weeks, two million Viagra" prescriptions had been written in the United States alone. The side effects of Viagra", now also sold under other names, include headaches, flushing, indigestion, and a temporary color-vision change. A small number of users have also suffered a heart attack or stroke.
