WASHINGTON — An investigation into whether Viagra and other popular impotence drugs may cause permanent eye damage is unlikely to lead to a recall but could prompt new warnings, federal officials and medical experts said yesterday.

The condition — nonarteritic anterior ischemic optic neuropathy, or NAION — involves damage to the optic nerve caused by an interruption of blood supply.

The Food and Drug Administration is looking into 38 reports of NAION in men who took Viagra, four involving Cialis and one with Levitra. Those are relatively small numbers, given that Viagra alone has been used by 23 million men worldwide since its approval in 1998, according to its maker, Pfizer.

"We haven't been able to determine that there is a cause and effect due to the medication," FDA spokeswoman Suzanne Trevino said. "But we are taking it seriously."

Most of the men who suffered eye damage did not go completely blind but lost part of their vision in one eye.

Complicating the question of whether the drugs are at fault, NAION is considered one of the most common causes of sudden vision loss in older people, and estimates suggest there are 1,000 to 6,000 cases a year. Risk factors include diabetes and heart disease, two of the leading causes of impotence.

Trevino said the FDA is talking with Pfizer about formally disclosing the risk in its prescribing literature for doctors. Cialis, which is a joint venture between Eli Lilly and Bothell biotech Icos, recently changed its information to acknowledge reports of eye damage.

Current warnings


Federal health officials are examining rare reports of eye damage among some men using impotence drugs.

The drugs also may have other rare side effects on vision. Warning labels for Viagra, Cialis and Levitra list such potential side effects, according to the drugs' Web sites.

Viagra: "Less common side effects that may occur are temporary changes in color vision (such as trouble telling the difference between blue and green objects or having a blue color tinge to them), eyes being more sensitive to light or blurred vision."

Cialis "may uncommonly cause ... vision changes, such as seeing a blue tinge to objects or having difficulty telling the difference between the colors blue and green."

Levitra "may uncommonly cause ... vision changes, such as seeing a blue tinge to objects or having difficulty telling the difference between the colors blue and green."

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Dr. Howard Pomeranz, a professor of ophthalmology at the University of Minnesota, reported a suspected link between Viagra and eye damage in a medical-journal article published in March.

"This particular eye problem should be added to the list of side effects that patients have access to," Pomeranz said yesterday. But, he added, the medical evidence at this point does not warrant withdrawing the drug.

"I'm not recommending that men stop taking this drug," he said. "It's very good for what it's prescribed for."

However, Pomeranz said, patients who have NAION in one eye should be strongly cautioned about the risk of using Viagra. And any patient who experiences blurriness or a loss of peripheral vision should stop taking such medications and consult a doctor immediately.

An eye exam could help patients learn whether they may be at risk. Doctors say one risk factor for NAION is tightly bundled nerves and blood vessels in the back of the eye. An eye doctor can identify that condition.

The reports of eye damage come at a time 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 yanked its pain reliever Vioxx from the market last year because of potentially deadly heart trouble.

Big money is at stake. Pfizer 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 eye-damage cases, Pfizer shares closed down 55 cents, to $28.35, yesterday on the New York Stock Exchange. Eli Lilly stock was down slightly, to $59.05 a share. Icos closed at $22.50, down 7 cents. Shares of Bayer, which makes Levitra in Germany, rose slightly to $34.28.

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 problems whose doctors have warned that sex itself could be too much exertion.

All three drugs 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.

In a statement, Pfizer said there was no evidence that NAION occurred more frequently in men taking Viagra than in men of similar age and health who did not take the drug.

"Most of the reported cases in which NAION has occurred in men taking Viagra have involved patients with underlying anatomic or vascular risk factors," the company said. "This makes it impossible to determine whether these events are caused by the patient's underlying risk factors, anatomical defects, Viagra, or a combination of these factors."

Eli Lilly spokeswoman Kindra Strupp said the company has updated its prescribing literature to reflect the reports of NAION. But "we don't see any cause-and-effect link," she added. "It certainly has not been established at this point."

Pomeranz, of the University of Minnesota, said he disagreed with the drug companies' assertion that any association between the medications and eye damage could be coincidental. In the 20 or so cases he reviewed, the onset of eye problems followed the patient's use of Viagra, usually by several hours but sometimes by as few as 45 minutes.

"It's really the time connection between when the patient took the drug and when he noticed the loss of vision that convinces me there's a link," Pomeranz said.

His journal article conceded that "a definite causal relationship cannot be established at this time." But in view of the growing use of such drugs, he wrote, the FDA must investigate.

The situation presents a common dilemma for the FDA: Although drugs may be tested on several thousand people before their approval, rare side effects may not emerge until many more have used the medication.

Some doctors said yesterday that patients inquiring about the potential risk of eye damage were not overly alarmed.

"It's ... way out of proportion," said Dr. Steven Schwartz, chief of the retina division at the Jules Stein Eye Institute at UCLA. He sent three cases to the FDA, but as in the other reports received by the agency, the patients had other risk factors for NAION.

Alonso-Zaldivar reported from Washington and Zarembo from Los Angeles.

Material from The Associated Press is included in this report.