It is not clear whether the deaths were caused by Viagra or whether the sexual activity made possible by the drug prompted heart attacks in men not accustomed to such vigorous behaviour. More than a million menhave taken Viagra since it went on the market in March, making it the fastest selling prescription drug in history.
Although Pfizer Pharmaceuticals maintains that there is no need to panic, news of the deaths may shrink demand for the small blue pills dubbed Magic Bullets.
A spokeswoman for Pfizer Pharmaceuticals said: "The company maintains that this is an effective and safe medicine when used appropriately by men who have diagnosed erectile dysfunction. Men with erectile dysfunction tend to be older and frequently suffer from serious underlying medical conditions ranging from diabetes to hypertension to cardiovascular disease."
All six deaths occurred in America, where doctors have been issuing 3,000 prescriptions for Viagra a week. The drug is available in Britain only in a few private clinics, where it is sold for pounds 17 after a consultation with a doctor. Hundreds of men are said to have flown from Europe to New York to get hold of the pills.