Pfizer is in discussions with the FDA about adding a disclosure to Viagra's label to say that in rare cases, men taking Viagra developed blindness. However, there is no proof that Viagra caused the blindness.

The Food and Drug Administration still is investigating, but has no evidence yet that the drug is to blame, said spokeswoman Susan Cruzan.

This type of blindness is called NAION, or non-arteritic anterior ischemic optic neuropathy. It can occur in men who are diabetic or have heart disease, the same conditions that can cause impotence and thus lead to Viagra use.

The FDA has 42 reports of the blindness, 38 among users of Viagra and four among users of Cialis. There were no cases reported among users of Levitra, the third impotence drug.

"We take this seriously," said FDA's Cruzan.

The FDA is in discussions about the reported cases with the manufacturers of the three drugs in case there is a problem with the class of medication and to see if changes need to be made to their labels.

'Every Drug Is Unique'

Eli Lilly & Co., which manufactures Cialis, refers to vision problems as an uncommon side effect, including seeing a blue tinge or having difficulty telling the difference between blue and green. "These are not all the side effects of Cialis," it says on its Web site.

Viagra manufacturer Pfizer Inc. also refers on its Web site to some vision issues: "Less common are bluish or blurred vision, or being sensitive to light. These may occur for a short time." That language had been available before the current inquiry.

Pfizer spokesman Daniel Watts confirmed Friday that the drugmaker was in discussions with the FDA about adding a disclosure to Viagra's label to say that in rare cases, men taking Viagra had developed blindness.

However, he said there is no proof that Viagra caused the blindness. He said that men who take Viagra often have high blood pressure and high cholesterol, which are also associated with the conditions that can cause blindness.

Levitra is sold in the United States by GlaxoSmithKline PLC and Bayer AG.

Viagra was approved by the government in 1998. It may aid in the treatment of enlarged hearts that can result from high blood pressure, tests on animals indicate.

Levitra was approved in August 2003, and Cialis in November of that year.

There have been no reports connecting Levitra to blindness, said Michael Flemming, a spokesman for GlaxoSmithKline PLC. "We are confident about the safety of our product," said Flemming.

Levitra, Cialis and Viagra all work in the same fashion, but Flemming said that doesn't mean they all have the same side effects. "Every drug is different. Every drug is unique," he said.

Downsides of Prescription Drugs

Analyst Jason Napodano of Zacks Investment Research Inc. said he doesn't think Viagra sales will stumble because of the new reports because the number of cases is so small. "It is too early to say that Viagra is causing this (the blindness) and look how small the numbers are," he said.

Viagra, approved to treat erectile dysfunction, 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.

The drug's label also warns of erections lasting longer than four hours, painful erections lasting longer than six hours, headache, flushed skin and vision problems.

Pfizer Inc. said in its most recent quarterly filing with the Securities and Exchange Commission that sales of the drug rose 5 percent -- to $438 million in the first quarter of the year. Pfizer also said at the time that Viagra had a 68 percent worldwide market share.

Viagra sales have been under pressure from new competitors Cialis and Levitra, with revenue sinking 11 percent last year to $1.68 billion from $1.88 billion. Pfizer shares fell in early trading on the New York Stock Exchange.

Thousands of Lawsuits

Doctors and patients have become increasingly wary of the downsides of prescription drugs after Merck & Co. yanked its pain reliever Vioxx from the market last year because of its potentially lethal side effects.

Merck potentially faces thousands of lawsuits over Vioxx and analysts have estimated its liability may reach $18 billion.

Pfizer was asked to remove its pain reliever Bextra from the market because of its side effects and sales of its other arthritis drug Celebrex are falling. Vioxx, Celebrex and Bextra all fall into a category of drugs known as Cox-2 inhibitors.

Industry critics have been demanding that drug companies conduct more post-marketing studies in order to do a better job of discovering side effects once drugs hit the market.

For more than two decades, Americans have mostly pressed for quicker approval of what they hope can be lifesaving drugs for such diseases as AIDS and cancer. But many now are wondering if medicines -- a $200 billion industry annually -- are coming out too fast and doing too much harm.

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