 Viagra
Overview and history
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 citrate.
The pills are blue 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.
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 smashing 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 (=1000 million).
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.
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.
Other drugs that operate by the same mechanism include Cialis and
Levitra.
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 include:
- 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 (including genetic
disorders of retinal phosphodiesterases)
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.
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 50mg and then lower
or raise the dosage as appropriate. The drug is sold in three dosages
(25mg, 50mg, and 100mg), all three costing about USD$10 per pill.
Investing in a pill cutter can therefore be advisable.
Some patients may find various degrees of discomfort at the 25 or 50mg
dosage level, including flushing of the face, head pressure, headache, and
tachycardia. For these patients a dosage of 10 mg to 12.5 mg may be more
appropriate yet effective.
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