Science Fair Project Encyclopedia
Sildenafil citrate, sold under the name Viagra, is a drug used to treat male erectile dysfunction (impotence), developed by the pharmaceutical company Pfizer. The pills are blue with the words "Pfizer" on one side and "VGR xx" (with xx being either 25, 50 or 100 as the dose of that pill in milligrams) on the other.
Viagra was initially developed to treat heart disease (angina). In 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 to consumers on TV (famously being endorsed by Bob Dole). Numerous sites on the Internet offer Viagra for sale after an "online consultation," a mere web questionnaire. It is likely that many men (and some women) experiment with the drug to increase sexual prowess or pleasure. Annual sales of Viagra in the period 1999-2001 exceeded $1 billion.
It has been suggested that Viagra would lead to a marked drop in the demand for certain traditional remedies, such as tiger penises and rhinoceros horns and that the drug may therefore help to preserve these endangered species. However, this is unlikely in that these parts of endangered species are not only used to treat impotence. Rhinoceros horns, for example, are used as a treatment for high fever. Further, since Viagra has not been shown to possess aphrodisiac properties, it is unclear that the natural remedies would compete with this new clinical drug.
The "Viagra" name has become so well known that many fake aphrodisiacs now call themselves "herbal Viagra" or are presented as blue tablets imitating the shape and colour of Pfizer's product.
Pfizer's worldwide patents on sildenafil citrate will expire in 2011 - 2013. The UK patent held by Pfizer on the use of PDE5 inhibitors (see below) as treatment of impotence has been invalidated in 2000 because of obviousness; this decision was upheld on appeal in 2002.
Viagra is also informally known as "Vitamin V".
The name "Viagra" is a marketing invention. It was possibly inspired by the Sanskrit word "vyāghra", which means "tiger". The word rhymes with "Niagara" (Niagara Falls is a popular honeymoon destination). The sound of the word also suggests the words "vigor" and "virile".
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 weight: 666.7 g/mol
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), 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 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.
Viagra is metabolised by hepatic enzymes and excreted by both the liver and kidneys. If taken with a high fat meal, there may be a delay in absorption of Viagra and the net effect might be muted slightly as the plasma concentration will be lowered.
Some reports have claimed that Viagra causes enhanced sexual pleasure for women by increasing blood flow to the sexual organs.
Contraindications and side effects
- 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
- Recent stroke or heart attack (myocardial infarct)
- Hereditary degenerative retinal disorders (including genetic disorders of retinal phosphodiesterases)
Common side effects include sneezing, headache, flushing, dyspepsia, prolonged erections, palpitations and photophobia. Visual changes including blurring of vision and a curious bluish tinge have also been reported.
Dosage and price
The dose of Viagra is 25mg to 100mg taken once per day between 0.5 to 4 hours before sexual intercourse.
It is usually recommended to start with a dosage of 50 mg and then lower or raise the dosage as appropriate. The drug is sold in three dosages (25, 50, and 100 mg), all three costing about USD$10 per pill. Viagra is not scored, meaning there is no guarantee that the drug is evenly distributed throughout the tablet, therefore it is not advisable to cut it to change dosage.
- Commercial competitors for Sildenafil citrate include:
- Dr. John R. Brinkley began a fad for finding cures for male impotence during the 1930s. He used the medium of radio to achieve the same kind of advertising boom to treat the same kind of symptoms. Today the medium of television is being used for that same purpose.
- Home Page for Viagra
- Pfizer Pharmaceutical (Company website) - manufacturer of Viagra; prescribing information available in PDF format.
- FDA Web Site for Viagra Consumer Information
- Viagra Misunderstood Article describing what Viagra feels like
- Cheitlin MD, Hutter AM Jr, Brindis RG, Ganz P, Kaul S, Russell RO Jr, Zusman RM. ACC/AHA expert consensus document. Use of sildenafil (Viagra) in patients with cardiovascular disease. American College of Cardiology/American Heart Association. J Am Coll Cardiol. 1999 Jan;33(1):273-82. (Medline abstract) (Full text)
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