Viagra was developed by and is being marketed by the pharmaceutical company Pfizer

Viagra acts by inhibiting cGMP specific phosphodiesterase type 5, an enzyme that regulates blood flow in the penis.

Since becoming available in 1998, Viagra has been the prime treatment for erectile dysfunction.

The primary competitors of Viagra on the market are tadalafil (Cialis UK) and vardenafil (Levitra UK).

The mechanism of action of Viagra involves the release of nitric oxide (NO) in the corpus cavernosum of the penis.

NO binds to the receptors of the enzyme guanylate cyclase in Viagra leading to smooth muscle relaxation (vasodilation) of the intimal cushions of the helicine arteries, resulting in increased inflow of blood and an erection.

Robert F. Furchgott won the Nobel Prize in Physiology or Medicine in 1998 for his discovery and analysis of endothelium-derived relaxing factor, a key part of the NO mechanism of action.

Viagra is a potent and selective inhibitor of cGMP specific phosphodiesterase type 5 (PDE5), which is responsible for degradation of cGMP in the corpus cavernosum.

The molecular structure of Viagra is similar to that of cGMP and acts as a competitive binding agent of PDE5 in the corpus cavernosum, resulting in more cGMP and better erections.

Without sexual stimulation, and therefore lack of activation of the NO/cGMP system, Viagra should not cause an erection. v Other drugs that operate by the same mechanism include tadalafil (Cialis UK) and vardenafil (Levitra UK) both cheaply available online from UKMeds4U.com.

Viagra is metabolised by liver enzymes and excreted by both the liver and kidneys.

If genuine Viagra is taken with a high-fat meal, absorption is reduced; the time taken to reach the maximum plasma concentration increases by around one hour, and the maximum concentration itself of Viagra is decreased by nearly one-third.

Viagra is now standard treatment for erectile dysfunction in all settings, including diabetes.

People on antidepressants may experience sexual dysfunction, either as a result of their illness or as a result of their treatment. A 2003 study showed that Viagra improved sexual function in men in this situation.

Following up to earlier reports from 1999, the same researchers found that Viagra was able to improve sexual function in female patients on antidepressants as well.

Professional sports players have been using drugs such as Viagra thinking that the opening of their blood vessels will enrich their muscles, therefore enhancing their performance.

Altitude sickness uses of Viagra has been shown to be useful for the prevention and treatment of High-altitude pulmonary edema associated with altitude sickness such as that suffered by mountain climbers.

While this effect has only recently been discovered, Viagra is already becoming an accepted treatment for this condition, in particular in situations where the standard treatment of rapid descent has been delayed for some reason.

As well as erectile dysfunction, Viagra is also effective in the rare disease pulmonary arterial hypertension (PAH).

Viagra relaxes the arterial wall, leading to decreased pulmonary arterial resistance and pressure.

This effect of Viagra in turn, reduces the workload of the right ventricle of the heart and improves symptoms of right-sided heart failure.

Because PDE-5 is primarily distributed within the arterial wall smooth muscle of the lungs and penis, Viagra acts selectively in both these areas without inducing vasodilation in other areas of the body.

Pfizer, manufacturers of Viagra, submitted an additional registration for Viagra to the FDA, and was approved for this indication in June 2005.

The preparation is named Revatio, to avoid confusion with Viagra and the 20 milligram tablets are white and round.

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