LOS ANGELES, July 8 (Xinhuanet)-- The US Food and Drug Administration (FDA) Friday ordered warnings onto the labels of Viagra and two other drugs that some users have developed a form of blindness.

    While cautioning that it's impossible to know if the pills are to blame for non arteritic ischemic optic neuropathy (NAION), a condition where blood flow is blocked to the optic nerve, the FDA said patients should stop taking Viagra, Cialis or Levitra when they feel vision loss.

    "FDA advises patients to stop taking these medicines, and call a doctor or healthcare provider right away if they experience sudden or decreased vision loss in one or both eyes," the FDA saidin a statement.

    "Further, patients taking or considering taking these products should inform their health care professionals if they have ever had severe loss of vision, which might reflect a prior episode of NAION. Such patients are at an increased risk of developing NAION again," the statement said.

    This move comes as the agency is under intense pressure to investigate more aggressively and warn the public about drug side effects. NAION is considered one of the most common causes of sudden vision loss in older people, with anywhere from 1,000 to 6,000 cases a year.

    Moreover, risk factors include diabetes and heart disease, two of the leading causes of impotence.

    The FDA has 43 reports of NAION among the impotence drug users,38 for Viagra, four for Cialis and one for Levitra. They include varying degrees of vision loss, including blindness.

    Those are rare numbers, given that Viagra alone has been used by 23 million men worldwide since its approval in 1998, according to drug maker Pfizer Inc.

    "At this time, it is not possible to determine whether these oral medicines for erectile dysfunction were the cause of the lossof eyesight, or whether the problem is related to other factors such as high blood pressure or diabetes, or to a combination of these problems," the FDA's statement noted.

    Cialis is marketed by Eli Lilly & Co. and ICOS Corp. Levitra is sold by GlaxoSmithKline PLC and Schering-Plough Corp. in the United States, and by Bayer AG elsewhere. Enditem