Shares in Pfizer, the American pharmaceutical giant, slumped almost 3 per cent today, after the US Food and Drug Administration admitted it is investigating whether Viagra, the company’s erectile dysfunction drug, was responsible for a number of cases of blindness.

The FDA statement came after a broadcast by CBS, the American television network, that there was a possible, but yet unproven, link between the drug and an unusual form of blindness called non-arteritic anterior ischemic optic neuropathy (NAION).

Susan Cruzan, a spokeswoman for the FDA, said the agency has 42 reports of the blindness, 38 among users of Viagra and four among users of Cialis, a rival anti-impotence drug.

In a statement to CBS, Pfizer maintained that Viagra was shown to be safe in clinical trials. The drugmaker also said that NAION has been linked to a variety of other risk factors, including diabetes, high blood pressure and high cholesterol.

Nonetheless Daniel Watts, spokesman for Pfizer, confirmed today that the drugmaker was in talks with the FDA to revise the labelling on Viagra to indicate its use might lead to blindness in rare cases.

The FDA disclosure comes at a time when the drug industry can ill afford negative publicity about another class of blockbuster medicines.

Last year Merck withdrew its pain reliever Vioxx from the market because of its potentially lethal side effects. Merck now faces thousands of lawsuits over Vioxx and analysts have estimated its liability may reach $18 billion (£9.8 billion).

Pfizer has also had to remove its pain reliever Bextra from the market because of its side effects and sales of its other arthritis drug, Celebrex, are falling.

The news also comes as some lawmakers in Washington are pushing to eliminate state funding for prescriptions of "lifestyle" drugs such as Viagra.

On its website, Pfizer already refers to some vision issues with Viagra.

"Less common are bluish or blurred vision, or being sensitive to light. These may occur for a short time," it says.

The website also warns that men with heart conditions or patients taking drugs that contain nitrates should not use Viagra. The drug’s label also warns of erections lasting longer than four hours, painful erections lasting longer than six hours, headache, and flushed skin.

Pfizer recently reported that sales of the drug rose 5 per cent to $438 million (£250 million) in the first quarter of 2005. The company also said that Viagra had a 68 per cent worldwide market share.

About 23 million men have used the drug worldwide.

Click here for CBS's online report and video

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