Clue to mystery Viagra deaths

Viagra has been linked to a number of deaths

The mystery behind the deaths of a small number of men taking the anti-impotence drug Viagra may have been solved by scientists.

Although Viagra has achieved worldwide renown as a highly effective treatment for impotence, it was originally developed to prevent heart attacks and strokes.

Our research suggests Viagra may present a risk for patients with pre-existing conditions

Professor Xiaoping Du

The drug was thought to work not only by dilating the blood vessels, but also by stopping tiny solid particles in the blood called platelets from clumping together and forming clots that could block blood flow and trigger a heart attack or stroke.

However, new work by researchers from the University of Illinois at Chicago has found that the drug in fact has the reverse effect - it stimulates the clumping of platelets, thus raising the risk of clotting.

Lead researcher Professor Xiaoping Du said: "Viagra, by itself, probably is not sufficient to cause a heart attack in healthy people, but our research suggests that it may present a risk for patients with pre-existing conditions such as atherosclerosis."

Effect on cells

The researchers tested the effect of Viagra on platelets taken from normal donors.

Alone the drug did not stimulate the platelets to stick together.

But it did if other compounds found in the bloodstream when the blood vessels are damaged - such as when somebody is suffering from heart disease - were added.

In fact, under these conditions Viagra caused the platelets to clump together at concentrations well below those found in patients prescribed the drug as an impotence treatment.

Viagra acts on platelets by inhibiting the breakdown of a compound found in cells called cyclic guanosine monophosphate (cGMP).

For 20 years scientists have thought cGMP acts to stop platelet clumping.

But the Chicago researchers believe that in fact cGMP initially causes platelets to clump together to seal a wound, only later reversing its effect to stop an excessive build-up that might block a blood vessel.

In most circumstances, they argue this would work well.

But a person at risk may already have blood vessels narrowed to the point that even the initial clumping of platelets could be enough to cause problems.

Safety record

Joel Morris, a spokesman for Pfizer, which makes Viagra, told BBC News Online that the drug had been shown to have a very impressive safety record.

"We take this research seriously, and it will be scrutinised by our medical teams.

"But a recent study by the Drug Safety Research Unit in the UK of 9,000 patients found that men who take Viagra are at no greater risk of heart problems than the general population."

It is estimated that 20 million men worldwide have taken Viagra.

The research is published in the journal Cell.