Bottles of the drug already carry a warning that it is potentially dangerous when taken in combination with nitrate drugs for heart disease.

But researchers, writing in the New England Journal of Medicine, warn that the risk to men with heart disease may be greater than envisaged.

They warn that Viagra may also be linked to a life-threatening lung complication and to bladder infections in women whose partners use the drug.

Irregular heartbeat

A Los Angeles doctor reported two cases of men with heart conditions who experienced problems after taking Viagra even though they were not taking nitrate drugs.

Both men suffered serious disruption to their heart rhythm after taking Viagra and having sex.

Research has shown that the risk of heart problems more than doubles in the two hours following sexual activity, and that one in three men with heart disease do not get enough oxygen to their heart muscle during sex.

Lung problem

Another report in the same journal described an 82-year old man who developed a fatal lung problem after taking three Viagra tablets over a two-day period.

The report said that Viagra should be used with care in people who might be predisposed to lung bleeding.

A third report found bladder infections in 15 of 100 women whose spouses received Viagra.

"Men treated with sildenafil (the scientific name for Viagra) should be advised to tell their female sexual partners to drink plenty of water and empty their bladders immediately after sex to avoid an infection," the report warned.

Last month, the U.S Food and Drug Administration reported that it had identified at least 69 deaths in the United States that might be linked to Viagra.

However, as in the cases cited in the New England Journal of Medicine, a direct cause-and-effect relationship has not been established.

Twelve of the 69 men had taken nitroglycerin or some other nitrate medicine and 18 died during or immediately after sex. Most had some type of risk factor for heart disease.

Worldwide, there have been at least 120 deaths linked to use of Viagra.

Doctors must be careful

Casualty staff should ask whether a patient is taking Viagra

Mr Roger Kirby, honorary secretary of the British Association of Urological Surgeons, said none of the 200 patients for whom he had prescribed Viagra had suffered any complications.

"The patients seem very happy with it, and there has been no problem," he said.

"But it is helpful that these reports are coming through. Doctors need to be careful, and this highlights the need for a doctor to prescribe Viagra. It should not be available over the counter until we are certain of the side effects."

Mr Kirby, a consultant urologist at St George's Hospital, London, said although clinical trials had been carried out on 4,000 patients, it was impossible to isolate all the side effects under experimental conditions.

He was concerned that patients who were taking Viagra might be treated with nitrate preparations if admitted to casualty suffering from heart problems.