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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 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 dose of that pill
in milligrams) on the other.
History
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.
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.
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.
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.
How Viagra works
During stimulation, the body releases chemicals causing
blood vessels to open up and supply more blood to the penis.
Enzymes break down these chemicals and allow an erection to
go down. Viagra blocks the action of those
enzymes. By blocking the enzyme, blood flow to the penis
increases, resulting in an erection. The drug will not
produce an erection at recommended doses without sexual
stimulation.
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