Viagra info
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. |
