The possibility of a link with blindness was raised earlier this year, when Dr. Howard Pomeranz reported in an ophthalmology journal on seven patients who reported NAION vision loss within 36 hours of a Viagra dose. 'A definite causal relationship cannot be established at this time,' he wrote.

The Food and Drug Administration disclosed Friday that it was in discussions with the makers of Viagra, Cialis and Levitra about what the labels of those drugs should say about the rare cases of varying degrees of vision loss, including blindness. The maker of Cialis already has voluntarily added a one-line mention to its label.

At issue is sudden vision loss when blood flow to the optic nerve is blocked, a condition called NAION or non-arteritic anterior ischemic optic neuropathy.

The FDA has 43 reports of NAION among the impotence drug users: 38 for Viagra, four for Cialis and one for Levitra, said spokeswoman Susan Cruzan.

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

Leading Causes of Impotence

Also complicating the question: NAION is considered one of the most common causes of sudden vision loss in older people, and estimates suggest there are anywhere from 1,000 to 6,000 cases a year. Risk factors include diabetes and heart disease, two of the leading causes of impotence.

Still, "we take this seriously," Cruzan said.

The questions come at a time when federal regulators and the drug industry are facing criticism about what they do to ensure the safety of drugs already on the market. Pressure on the FDA to investigate reports of side effects has increased since Merck & Co. yanked its pain reliever Vioxx from the market last year because of potentially deadly heart trouble.

Big money is at stake. Pfizer Inc. said in its most recent quarterly filing with the Securities and Exchange Commission that sales of Viagra rose 5 percent - to $438 million - in the first quarter of the year.

After slumping more earlier in the day after news of the blindness cases, Pfizer shares closed down 55 cents, or 1.9 percent, at $28.35 Friday on the New York Stock Exchange.

Temporary Vision Changes

Viagra and its competitors are blockbuster drugs that revolutionized treatment of erectile dysfunction, and they already come with serious warnings: They're not to be used by men who take nitrate-containing drugs, because the interaction could cause deadly drops in blood pressure, or by men with heart conditions whose doctors have warned that sex itself could be too much exertion.

All three also warn about temporary vision changes -- seeing bluish tinges or having difficulty distinguishing between green and blue. The drugs apparently have a temporary effect on the retina, a different issue from NAION.

Viagra also is at the center of controversy over Medicaid's payment for prescriptions of the drug for convicted sex offenders in New York and other states.

Definite Causal Relationship Cannot Be Established

The possibility of a link with blindness was raised publicly earlier this year, when Dr. Howard Pomeranz of the University of Minnesota reported in an ophthalmology journal on seven patients who reported NAION vision loss within 36 hours of a Viagra dose.

"A definite causal relationship cannot be established at this time," Pomeranz wrote.

Viagra and its competitors work by slightly dilating arteries so that blood flow in the penis increases. Whether it affects blood flow to the eye isn't known, but Pomeranz argued that some effect on the optic nerve is plausible.

So he urged that ophthalmologists ask NAION patients whether they use impotence drugs, and report any additional cases. Also, Viagra users who suffer NAION in one eye should be cautioned that continued use might raise the risk of vision loss in the other eye, Pomeranz wrote.