May 27 (Bloomberg) -- Pfizer Inc., the world's largest drugmaker, said U.S. regulators may require a label warning that the Viagra impotence drug has been linked to reports of blindness and vision loss in at least one in a million users.

Pfizer has received 23 reports of such side effects since Viagra was introduced in 1998, spokesman Daniel Watts said today in an interview. The New York-based company is in talks with the Food and Drug Administration to update Viagra's label to ``reflect those rare occurrences,'' Watts said. A warning was added to the label of a rival drug, Cialis, this month.

Viagra, the first pill to treat erectile dysfunction, has been taken by 23 million men and generated $1.68 billion in sales last year. Pfizer's revenue already was hurt when its Celebrex pain drug was linked to heart risks and the related Bextra was withdrawn after being tied to a fatal skin disorder. Chief Executive Hank McKinnell also faces generic competition in the next two years on drugs that account for a third of sales.

``It's an extraordinarily rare complication -- perhaps of the underlying condition that the patient has,'' said Deutsche Bank Securities analyst Barbara Ryan, who has a ``buy'' rating on Pfizer and doesn't own the shares, in an interview today. ``I don't personally think it's that significant.''

Pfizer shares fell 64 cents, or 2.2 percent, to $28.26 at 2:02 p.m. in New York Stock Exchange composite trading. Before today, they had risen 7.5 percent this year.

Class Effect?

Viagra users may be prone to vision loss already because illnesses that cause erectile dysfunction, such as diabetes and high blood pressure, also are linked to sight problems, Pfizer's Watts said.

Cialis, sold by Eli Lilly & Co. and Icos Corp., works in a similar way to Viagra. The companies changed the Cialis label within the past two weeks to say there have been three reported cases of non-arteritic anterior ischemic optic neuropathy, a cause of blindness, in users of the medicine. Five million men have been prescribed Cialis worldwide, the company said.

``We did it voluntarily because we thought it was important to make health-care practitioners and prescribers aware,'' Lilly spokesman Dan Collins said. ``This is not the result of the FDA asking us to look into this.''

Prescriptions for Viagra, which costs about $10 a pill, account for about 67 percent of the U.S. market for erectile- dysfunction drugs, with Cialis trailing at 22 percent, according to data compiled by the health information company Verispan. Levitra, sold by Bayer AG and GlaxoSmithKline Plc, with Schering- Plough Corp. as marketing partner, accounts for 11 percent of prescriptions.

FDA Comments

The FDA has 43 reports of varying degrees of vision loss, including blindness, among users of impotence drugs: 38 among Viagra users, four among Cialis users and one among Levitra users, spokeswoman Susan Cruzan said today in Rockville, Maryland. The condition is not uncommon in men who have high blood pressure and diabetes, Suzanne Trevino, another FDA spokeswoman, said.

``The FDA is working with the company,'' Trevino said. ``People who have concerns should talk with their doctors.''

The U.S. Congress has been scrutinizing how well the FDA monitors drug safety after Merck & Co. withdrew its $2.6 billion painkiller Vioxx in September because of links to heart attacks and strokes. A study published in 2000 had detected the heart risk, yet Vioxx's label wasn't changed until 2002.

Viagra's label already cautions users about possible side effects involving eyesight, such as increased sensitivity to light or blurred vision. There also have been reports of increased pressure in the eye and retinal vascular disease in people using the medicine, the label says.

Published Article

University of Minnesota researcher Howard Pomeranz yesterday spoke with CBS News about Viagra users suffering vision loss. Pomeranz in March published an article about seven cases of the side effects seen in men using the drug.

The men were between ages 50 and 69 and lost vision because of nonarteritic ischemic optic neuropathy after using Viagra, according to the article Pomeranz wrote in the March issue of the eye specialist publication, J Neuro-Ophthalmol. All of the men had conditions such as high blood pressure, diabetes and high cholesterol.

``That got us into discussions with the FDA about some change in wording in the post-marketing of our label to give more information to our patients,'' Michael Berelowitz, Pfizer vice president of worldwide medical, said in an interview. ``We'll follow this through to its logical conclusion.''

Public Citizen

The FDA should be in talks with the makers of competing impotence treatments Cialis and Levitra because they all work in a similar way, said Sidney Wolfe, the director of the consumer- advocacy group Public Citizen's Health Research Group, in a telephone interview.

``There is not any reason to think that the odds of this occurring are not exactly the same with all of these drugs,'' Wolfe said. ``Unless the FDA is really off the mark, they should be having discussions with all of the companies that make these products.''

Schering-Plough spokesman Mark Scampoli didn't immediately return a call seeking comment.

George Fleming, a Houston lawyer who represented people who once used Wyeth's withdrawn diet drugs, said it might be tough to prove a connection between Viagra and vision loss because the form of nerve damage occurs in people with hardened arteries and other conditions that can effect blood flow.

``The weak link can be the causation element,'' Fleming said in an interview. ``From a legal standpoint, it doesn't just take association. It takes causation.''

To contact the reporters on this story: Juliann Walsh in Princeton at jwalsh10@bloomberg.net; Kerry Dooley Young in Washington at kdooley@bloomberg.net.

Last Updated: May 27, 2005 14:07 EDT