Viagra

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Viagra is a drug used to treat male erectile dysfunction (impotence), developed by the pharmaceutical company Pfizer. The generic name for this compound is sildenafil. It is a blue pill with the words "Pfizer" on one side and "VGR xx" (with xx being either 25, 50 or 100 as the miligram dose of that pill) on the other.

Chemical name: 1-[4-ethoxy- 3-(6,7-dihydro- 1-methyl- 7-oxo- 3-propyl- 1H-pyrazolo [4,3-d]pyrimidin-5-yl) phenylsulfonyl]- 4-methylpiperazine citrate. Molecular formula: C22H30N6O4S.C6H807. MW: 666.7

Mechanism of action
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) which results in smooth muscle relaxation in the corpus cavernosum which leads to increased inflow of blood which leads directly to 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 on board, 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.


Contraindicated for:

- when taking other nitric oxide donors, organic nitrites and nitrates (which includes glyceryl trinitrate, sodium nitroprusside, amyl nitrite)
- in men for whom sexual intercourse is inadvisable due to cardiovascular risk factors
- severe hepatic impairment
- severe impairment in renal function
- [hypotension]
- recent [stroke] or heart attack (myocardial infarct)
- hereditary degenerative retinal disorders (incl genetic disorders of retinal phosphodiesterases)

The dose of viagra is 50mg to 100mg taken once per day between 0.5 to 4 hours before sexual intercourse. If taken with a high fat meal, there may be a delay in absorbtion of Viagra and the net effect might be muted slightly as the plasma concentration will be lowered.

Amongst viagra's serious adverse effects are: priapism, severe hypotension, myocardial infarction, ventricular arrhythmias, sudden death, stroke and increased intraocular pressure.

Common side effects include headache, flushing, dyspepsia, prolonged erections, palpitations and photophobia. Visual changes including blurring of vision and a curious bluish tinge have also been reported.

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