AN INVESTIGATION has been launched into the health risks of Viagra, the popular anti-impotence drug, amid concerns that it has caused blindness in up to 50 people.

US drugs regulators announced yesterday that they are examining a number of cases in which users have suffered optic neuropathy — known as “stroke in the eye”.

Health officials in Britain said last night that they had received reports of six suspected cases of blindness in similar circumstances — when blood flow to the optic nerve is cut off. The condition can lead to irreversible loss of vision.

Since its launch in 1998, Viagra has proved a global sensation. More than 23 million men have admitted to using it, and licensed and illegal trade on the internet, which allows men to buy the drug anonymously, accounts for tens of millions more sales.

More than $4 billion (£2.2 billion) was cut from the share value of Pfizer, Viagra’s manufacturer, on Wall Street yesterday as news of the potential problem sparked panic selling. News of the investigation comes after a succession of high-profile drug withdrawals.

Officials from the US Food and Drug Administration (FDA) announced that they were investigating 42 reports of blindness, 38 among users of Viagra and four among users of Cialis, one of its leading competitors. Non-arteritic anterior ischemic optic neuropathy (Naion) can occur in men who are diabetic or have heart disease — the same conditions that can cause impotence and lead to Viagra use.

Susan Cruzan, an FDA spokeswoman, said that the investigation was continuing but that there was no concrete evidence that the drugs were to blame. The FDA was in touch with the manufacturers of Levitra, another drug for erectile dysfunction, in case there was a problem with the entire class of medication. There have been no cases of blindness reported by Levitra users so far.

Pfizer said that it was aware of rare cases of blindess in Viagra users and that the company was in talks with regulators to change warnings on its label.The company also emphasised that no proof existed that blindness was linked to the drug.

Pfizer holds 68 per cent of the world market in anti-impotence drugs, but faces strong competition from Eli Lilly, which makes Cialis, and Levitra’s manufacturers, Bayer AG and GlaxoSmithKline. An FDA source told The Times that, in some of the cases, users took Viagra before going to bed and woke up with little or no vision the next morning.

“The patients may be old, they may have other health problems, but it is the time between them taking the drug and losing their vision that is of concern,” the source said.

Pfizer is facing at least one lawsuit from a Viagra user for loss of sight. Jimmy Grant, 57, has described how he felt pressure in his temple, the colour changed in his vision and lost all sight in his right eye. No connection was made by doctors. After taking the drug for three years, Mr Grant went blind in his left eye as well.

Several experts have recently raised concerns about the possibility that Viagra might be linked to Naion. They suggested that all men suffering from the eye condition should be asked if they had taken Viagra.

Pfizer said that a review of 103 clinical trials involving 13,000 patients had found no reports of the condition and that reports of loss of vision were “extremely rare”.

The Medicines and Healthcare Products Regulatory Agency in Britain said that five of the six British cases, who are all over the age of 50, had concurrent illnesses that may have contributed to their blindness.

Pfizer advises that possible side effects of Viagra include “bluish or blurred vision, or being sensitive to light” which “may occur for a short time”.

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