Study links Viagra to increased stroke risk.
By David Derbyshire, Science Correspondent.
12:01AM GMT 10 Jan 2003.
Viagra can promote blood clots and may be dangerous for people already at risk of heart disease and stroke, researchers said yesterday.
The findings could explain incidents of fatal cardiovascular disease in a small number of men taking the drug, they said.
However, because the study was carried out on blood cells in a test-tube rather than in people, others urged caution.
Pfizer, the company that makes Viagra, said there was no evidence that the drug, taken by 20 million men worldwide, increased the risk of heart disease or stroke.
Viagra was originally developed as a treatment for angina. It was designed to dilate blood vessels and prevent platelets, a type of blood cell, from clumping and forming clots.
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It was only when men taking part in a clinical trial reported unexpected side-effects that Pfizer realised it could treat erectile dysfunction.
However, a study published today in the science journal Cell suggests that Viagra can encourage platelets to clump together by elevating levels of a compound called cGMP.
The finding challenges 20 years of scientific assumptions that cGMP prevents platelet aggregation.
Dr Xiaoping Du, a pharmacologist at the University of Chicago College of Medicine, said: “Viagra by itself 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.”
Dr Du forced laboratory cells to make two proteins key to platelet clumping – one that helps platelets stick together and another that activates the first protein. They then isolated and studied the molecules that trigger these proteins.
It had long been accepted that cGMP inhibited the clumping of platelets but, to Dr Du’s surprise, cGMP also seemed to encourage clumping. The researchers believe that cGMP initially causes platelets to stick together to seal a wound, but later reverses to stop an excessive build up of cells that could block a blood vessel.
Viagra works by inhibiting an enzyme that breaks down cGMP. In tests Viagra alone did not promote clumping of platelets. But if there were small amounts of other chemicals typically found in damaged blood vessels, Viagra promoted clumping.
Doctors do not prescribe the drug to people with low blood pressure, recent strokes or heart attacks, or who are taking some drugs for heart disease.
A spokesman for Pfizer said there was no evidence that the drug caused heart attacks or stroke.
“In the clinical trials there were men with all sorts of health status and no increase in cardiovascular events,” he said. “We are confident that it is safe.”
A study of nearly 9,000 users in Britain last year found that the drug did not increase the risk of heart attacks.
Fiona Kirkwood, of the British Heart Foundation, said: “Men who have been prescribed nitrates should not be taking Viagra and there may be safety issues for men taking Viagra who have recently had a heart attack or stroke.
“However, so far there have only been a small number of stroke or cardiovascular events in men taking Viagra, and it is still unclear if there is a link between the two.
“Further large-scale research is needed.”
Cell Online [subscription required]
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