Shares of

Pfizer

(PFE) - Get Report

dropped Friday, a day after a television news report said the Food and Drug Administration was examining whether the impotence drug Viagra was linked to some cases of vision loss.

The FDA confirmed on Friday that it was investigating the matter, and in turn Pfizer confirmed it was talking to the agency "to update the Viagra label to reflect these rare ocular occurrences."

The company said it has reviewed 103 clinical trials involving 13,000 men and has found "no reports" of the disease called non-arteritic anterior ischemic optic neuropathy, or NAION. "Outside of clinical trials, Viagra has been used by more than 23 million men worldwide over the past seven years and reports of visual field loss due to NAION are extremely rare," Pfizer said.

The company added that "there is no evidence showing NAION occurred more frequently in men taking Viagra than men of similar age and health who did not take Viagra."

Proceed With Caution

CBS News

said Thursday that the FDA and Pfizer were talking about Viagra and NAION, reporting that the agency was checking about 50 cases of the eye disease linked to Viagra use. The

Associated Press

said Friday that the FDA was looking at 38 reports of NAION linked to Viagra.

By early Friday afternoon, Pfizer's shares were down 64 cents, or 2.2%, to $28.26, but several analysts cautioned investors about overreacting.

"While we expect the FDA to take these cases seriously, we do not believe they warrant concern with the product at this time," says Albert Rauch, of A.G. Edwards, in a Friday report to clients.

Rauch says NAION afflicts two to 10 of every 100,000 people over 50 years old. That's a higher rate than the 38 possible NAION sufferers among the 23 million-plus Viagra users. Rauch has a hold rating on the stock. He doesn't own shares, and his firm has a noninvestment banking relationship with the company.

"We do not believe that the FDA has evidence that Viagra is associated with

NAION," says John T. Boris, of Harris Nesbitt, in a Friday research note. "NAION is the most common acute optic nerve disease reported in men over 50 years of age and is caused by cardiovascular disease and not the acute use of Viagra," he says.

Many factors contributing to NAION -- high blood pressure, high cholesterol and diabetes -- also lead to erectile dysfunction, says Boris, who reaffirmed his outperform rating. He expects the FDA to request that Pfizer update the Viagra label "to include infrequent reports of blindness." His firm doesn't have an investment banking relationship. His research report says either the analyst, an associate or household member of the analyst or associate owns shares.

Assessing Side Effects

The

CBS News

report included references to research by Dr. Howard Pomeranz, associate professor of ophthalmology at the University of Minnesota Medical School. He published an article in March in the

Journal of Neuro-Opthalmology

in which he identified seven patients who had exhibited "typical features" of NAION, often described as a stroke of the eye, within 36 hours of taking Viagra.

"Six patients had vision loss within 24 hours after use of the agent," says an abstract of the journal article. "Final visual acuity in the affected eye ranged from 20/20 to light perception. Both eyes were affected in one individual."

Pomeranz noted that all of these men had high blood pressure, diabetes or high cholesterol. He added that each of the patients a had "low cup to disk ratio," a condition in which blood vessels and nerves "are tightly bundled together into the small space in the back of the eye," says a March 21 press release from the University of Minnesota.

"Viagra regulates a chemical in the body to constrict the arteries," Pomeranz said when his article was published. "This constriction may cut off the blood flow to the optic nerve, especially in people with a low cup to disk ratio." He concluded that Viagra "may provoke" NAION in certain high-risk patients. He said seven other cases had been reported.

Pfizer said Friday that most of the men experiencing NAION after taking Viagra had those risk factors identified by Pomeranz. "This makes it impossible to determine whether these events are caused by the patient's underlying vascular risk factors, anatomical defects, Viagra or a combination of these factors -- or to other factors," the company said in a statement.

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Approval Process

Viagra was approved by the FDA in 1998 based on clinical trials involving 3,700 people ages 19 to 87. Over the years, based on post-marketing surveillance, Pfizer has toughened the Viagra label warning primarily to reflect concerns about potential heart risk and about patients who use certain heart disease drugs that contain nitrates.

"There is a potential for cardiac risk of sexual activity in patients with pre-existing cardiovascular disease," says the Viagra label. "Treatments for erectile dysfunction, including Viagra, should not be generally used in men for whom sexual activity is inadvisable because of their underlying cardiovascular status."

The Viagra clinical trials also reported a host of other side effects, including headache (16%), flushing (10%) and stomach pain (7%).

Three percent reported "abnormal vision," which Pfizer describes as "mild and transient, predominantly color tinge to vision, but also increased sensitivity to light or blurred vision." The company adds that only one patient dropped out of a clinical study due to "abnormal vision."

The label says that although some men experience a blue/green tinge to their vision after taking the drug, "the finding is consistent with the inhibition" of a certain enzyme that affects the retina. At doses up to twice the recommended amount, Pfizer found "no effects of Viagra on visual acuity" or eye pressure.

But Pfizer also detected other side effects based on clinical trials, adding that "a causal relationship to Viagra is uncertain" because there were so few reports of these adverse reactions. The problems ranged from chest pain to vomiting and from tremor to vertigo. They also included conjunctivitis, eye pain, cataract, dry eyes and eye hemorrhage.

Post-marketing reports indicated an unspecified number of side effects including "temporary vision loss/decreased vision," ocular burning, ocular swelling or pressure and bloodshot appearance.

The company has spent much of its time in recent years trying to fight off competition from newer erectile dysfunction drugs -- Cialis from

Icos

(ICOS)

and

Eli Lilly

(LLY) - Get Report

and Levitra from

GlaxoSmithKline

(GSK) - Get Report

and

Bayer

-- as well as illegal versions of Viagra.

Earlier this year, Pfizer and

Microsoft

(MSFT) - Get Report

sued a pair of what they called "international pharmacy spam rings" to prevent them from selling illegal Viagra over the Internet.

According to the

Associated Press,

the FDA is checking reports four men taking Cialis and one man taking Levitra had experienced NAION and said the agency is talking to the makers of these products.