The Food and Drug Administration (FDA) says it agreed with the manufacturer, Pfizer, on extra wording on the drug's label, after reports of deaths and severe side-effects among Viagra users.

While stressing that it still believes Viagra is safe, the FDA says the new warnings make clear that patients with a history of particular health problems should be carefully examined by a doctor before they are prescribed the drug.

Since Viagra went on the market in April, 69 men have been reported to have died after taking it.

"The revised labelling addresses post-marketing reports of heart attacks, sudden cardiac deaths and hypertension," the FDA said in a statement.

History of heart problems or low blood pressure is seen as a risk factor

It said that many groups had been left out from the original safety trials for Viagra, including men who had suffered heart attacks, stroke or dangerous arrhythmia within six months, men with dangerously high or low blood pressure, men with heart failure or unstable angina (chest pains), or retinitis pigmentosa, an eye disorder.

"The new labelling notes that if Viagra is prescribed for these patients, it should be done with caution," the FDA said.

It also noted that patients who have heart disease are at risk from sexual activity anyway and have to be especially careful if they use Viagra.

The new label also warns of the risk of priapism - a prolonged and painful erection - and advises patients to seek prompt medical attention if their erection lasts longer than four hours.

Plane crash connection investigated

Meanwhile, accident investigators in the United States say they are trying to determine whether Viagra contributed to a plane crash in which the pilot - a Hollywood actor, William Knight - was killed.

Mr Knight, 56, who has appeared in films such as Wall Street and Born on the Fourth of July, was killed when his experimental light aircraft hit the water during a difficult manoeuvre.

A police spokeswoman has confirmed that officials of the National Transportation Safety Board have asked a medical examiner to determine whether Mr Knight - an experienced pilot - was using Viagra.

The American aviation authorities have advised pilots against using the drug within six hours of flying, because its side-effects can include difficulty in distinguishing between blue and green, which are used extensively in airport lighting and cockpit instruments.

During testing, Pfizer reported that 3% of men reported visual disturbances that ranged from sensitivity to light to seeing a bluish-green haze.