BEIJING, May 30 -- US regulators have received more than 40 reports of a type of blindness in men who use Viagra and other impotence drugs, but have not determined if the medicines were responsible.

    The Food and Drug Administration (FDA) said it has about 38 reports of the rare condition among Viagra users, four reports among users of Eli Lilly and Co.'s Cialis and one report in a man who took GlaxoSmithKline Plc's Levitra.

   In some cases users are getting visual problems within a short time.

    While it has been known for years that users of impotence drugs could experience short-term vision changes, seeing green or blueish hues, what has raised the red flags is the immediacy of the drug's impact and its lasting effect.

   They say the vision loss -- known as non-arteritic anterior ischemic optic neuropathy (NAION) -- is caused by a sort of stroke affecting the blood vessels of the eye when they become choked off, eventually causing some of the cells in the eye to die.

    Interestingly, the number of people who get this same type of stroke is actually about four times higher for people who don't take Viagra, the study's researchers say.

    New York-based Pfizer, Viagra's manufacturer, said it is weighing a change to the Viagra label. But it emphasized no proof exists that links the blindness to the drug, which was introduced in 1998 and had $1.68 billion in sales in 2004.

    More than 23 million men worldwide have taken Viagra over the past seven years, Pfizer said. Reports are extremely rare of the condition of NAION, the company said. NAION can cause permanent vision loss in one or both eyes.

   Pfizer already warns the drug should not be used by men with heart conditions whose doctors have warned them not to have sex.

    Also, patients taking drugs that contain nitrates have been warned not to take Viagra because of sudden, unsafe drops in blood pressure.

    (Agencies)