Sildenafil citrate, sold as the drug Viagra, is used to treat male erectile dysfunction (impotence), developed by the pharmaceutical company Pfizer.

Viagra was initially developed to treat heart disease (angina). However, during the trial studies, the penile erection enhancing effects were noticed. The drug was patented in 1996, approved by the FDA on March 27, 1998 (becoming the first pill approved to treat erectile dysfunction in the United States) and offered for sale in the United States later that year. It soon became a great success. Even though Viagra is only available by prescription from a doctor, it was advertised directly on TV . Annual sales of Viagra in the period 1999–2001 exceeded $1 billion.

Viagra works becuase part of the physiological process of erection involves the release of nitric oxide (NO) in the corpus cavernosum. This then activates the enzyme guanylate cyclase which results in increased levels of cyclic guanosine monophosphate (cGMP), leading to smooth muscle relaxation in the corpus cavernosum, resulting in increased inflow of blood and an erection. Sildenafil is a potent and selective inhibitor of cGMP specific phosphodiesterase type 5 (PDE5) which is responsible for degradation of cGMP in the corpus cavernosum. This means that, with Viagra, normal sexual stimulation leads to increased levels of cGMP in the corpus cavernosum which leads to better erections. Without sexual stimulation and no activation of the NO/cGMP system, Viagra should not cause an erection.

Formal Chemical Name (IUPAC)
1-[4-ethoxy-3-(6,7-dihydro-1-methyl- 7-oxo-3-propyl-1H-pyrazolo[4,3-d]pyrimidin -5-yl)phenylsulfonyl]-4-methylpiperazine

References

http://www.viagra.com/

http://www.rxlist.com/cgi/generic/viagra.htm