From May 16 issue of New Scientist - What goes up...
By Nell Boyce, Washington DC
the backlash against viagra has started. A
scientist involved in ongoing clinical trials of the first
pill for erectile dysfunction claims the reality doesn't
match the hype surrounding the drug. More
worryingly, ophthalmologists are concerned that
long-term use could damage eyesight.
Viagra dilates blood vessels, allowing more blood to
flow into the penis. Since its approval by the US Food
and Drug Administration (FDA) on 27 March, Viagra
has achieved huge sales for its manufacturer, Pfizer
of New York, and saturation media coverage. "The
hype that came out was incredible," says James
Barada of the Center for Male Sexual Health in
Albany, New York.
The clinical studies that convinced the FDA to
approve Viagra are published this week in The New
England Journal of Medicine (vol 338, p 1397).
Viagra significantly improved the quality of men's
erections and quadrupled their success at having
intercourse. In one study of 532 men taking either
Viagra or a placebo, those receiving 100 milligrams of
Viagra doubled their frequency of erections. Among
men taking this dose of Viagra, 69 per cent of the
attempts at intercourse were successful, compared
with only 22 per cent in the placebo group.
But many of the men in these studies had achieved
intercourse in the previous three months, and so were
not clinically impotent, ( Gee, now where have i heard this reported before??) according to Barada. He is
now running an eight-week study of Viagra, funded
by Pfizer, to see how it compares with other
impotence treatments.
Barada has written over 150 prescriptions for Viagra
and doesn't dispute that it improves the erections of
most men who take it. But of those who are
genuinely impotent, rather than merely lamenting the
loss of youthful vigour, only a third achieve sufficient
hardness to have intercourse after taking the drug. "I
consider them Viagra failures," says Barada. "You
don't take a drug just because it helps a little bit."
Since Viagra doesn't treat the underlying causes of
impotence, the condition will probably continue to
worsen until even the highest recommended doses
can't help, Barada adds.
Viagra also causes side effects, among them
headaches and fainting due to dilation of blood vessels
in other parts of the body. These symptoms occur
more often as the dose increases. Most men end up
taking 100 milligrams of the drug, the highest
recommended dose, but there are fears that some are
already taking higher doses despite their doctors'
warnings.
The most worrying side effect is blue-tinted vision in
some men taking Viagra. "Blue vision is a very
unusual symptom," says Michael Marmor, an eye
specialist at Stanford University in California, who
has seen Pfizer's data on Viagra.
(This also sounds vaguely familiar:)
Viagra works in the penis by inhibiting an enzyme
called phosphodiesterase. A similar enzyme exists in
the cone cells responsible for colour vision in the
retina. "Somehow, the way this drug blocks the
enzyme in the cone cells leads to the perception of
blue," says Marmor.
This has ophthalmologists worried, because people
with congenitally abnormal phosphodiesterase suffer
irreversible damage to their retinas over time. What's
more, a common cause of impotence is diabetes,
which can also damage the retina. Pfizer has issued a
statement saying it has "conducted rigorous visual
function tests at doses well above those
recommended for Viagra. These studies showed no
clinically significant effect on vision in either the short
or long term."
But the American Academy of Ophthalmologists
wants Pfizer to conduct more studies into how Viagra
affects vision. "The company has not measured the
electrical activity of the cone cells in the long term,"
says Marmor. This could be done using contact lenses
containing tiny electrodes capable of recording the
retina's electrical activity.
newscientist.com. |
