American class-action lawyers who specialise in suing consumer drug companies on behalf of patients killed or made ill by the side-effects of drugs, told The Times that they were ready to start building a case against Pfizer.

Pfizer revealed last week that a small number of men who had taken the drug to cure impotence had been struck with “optic neuropathy”, a condition also known as a “stroke in the eye”, which can lead to partial or total blindness.

The Food and Drug Administration, America’s medicines watchdog, is investigating the claims and is in talks with Pfizer about changing the warning label on Viagra to reflect the discovery.

Neither the company nor the FDA has concluded that the drug causes the condition, however.

So far fewer than 50 people are known to have been struck with the possible side-effect, but lawyers are consulting their own ophthalmic specialists and seeking patients who may have had their vision damaged by Viagra in an attempt to build a suit against Pfizer that could include thousands of patients.

Bill Federman, a class action lawyer based in Oklahoma City, said: “The issue here is that the public was never told that there was a potential ocular problem in taking Viagra, not a serious one such as is being claimed here.”

Pfizer has always warned on Viagra packaging that taking the drug can lead to blurred vision, or to vision tinted with a bluish hue.

Mr Federman is already suing Merck, the maker of Vioxx, a painkiller that was withdrawn from the market after it was shown to cause fatal heart attacks and strokes in thousands of patients who took it.

“But we have so little to go on with Viagra,” he said. “There have been no real independent studies of Viagra that focus on these kinds of ocular side-effects so that is was we must have first.”

Mr Federman said: “The Food and Drug Administration needs to release to the public all the information it has received about the potential dangers of Viagra. Only then can decisions about taking it be made.”

If the lawyers succeed in building a class-action case against Pfizer, the number of people included would be vast.Since its launch in 1998 more than 23 million men have taken the little blue pill, whether prescribed by a doctor or bought from unauthorised suppliers on the internet.

The damage of any Viagra lawsuit is potentially devastating to Pfizer. In the first three months of this year alone, the company sold $438 million (£240 million) of the drug.

Meanwhile on Friday more than $4 billion was cut from Pfizer’s market value as news of the potential Viagra problem sparked a panic sell-off of its shares. US stock markets were closed yesterday in observance of Memorial Day.

The company is already being sued by thousands of patients who took its Celebrex painkiller.

GlaxoSmithKline, the UK manufacturer of Levitra, a drug similar to Viagra, and Lilly ICOS, the maker of Cialis, another erectile dysfunction treatment, could also be hit by any Viagra lawsuit. The FDA is already looking into a small number of blindness cases in patients who took Cialis.

Both companies have already changed the warning labels on their drugs after the Viagra accusations were made public.

One user is already suing Pfizer because he lost the sight in one eye after taking Viagra. Jimmy Grant, 64, began taking it in 1998. In 2001 he began to lose his sight and has since gone blind in his right eye.

FIRMS’ LEGAL HEADACHES

US drug class-actions under way

• Accutane (acne treatment) Hoffman-La Roche
• Celebrex (painkiller) Pfizer
• Baycol (cholesterol treatment) Bayer
• Vioxx (painkiller) Merck
• Zyprexa (anti-depressant) Lilly ICOS

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