While the FDA has concluded that no causal relationship between Viagra and NAION has been established, Pfizer has agreed to include on the Viagra label that these reports exist in order to inform physicians and patients.

"After more than 10 years of rigorous clinical study and worldwide safety monitoring of a medicine used by more than 27 million men, there is no evidence to suggest a link between Viagra and blindness or other serious ocular events," said Joseph Feczko, Pfizer's chief medical officer.

"In addition, several studies specifically looking into the effect of Viagra in the eye by Pfizer as well as independent ophthalmologic experts found Viagra to have no serious adverse effects on the eye."

Common Risk Factors

Pfizer noted that NAION, or non-arteritic anterior ischemic optic neuropathyone, is one of the most common acute optic nerve diseases in older adults, and shares a number of common risk factors with erectile dysfunction: over 50 years of age, high blood pressure, high cholesterol and diabetes.

Most of the reported NAION cases occurred in Viagra users with underlying anatomic or vascular risk factors associated with the development of NAION, Pfizer said. The Food and Drug Administrationhad been looking into about 50 reports of Viagra patients who experienced vision loss.

No Causal Relationship

The drug maker is continuing talks with the FDA to update the Viagra label to reflect rare reports of NAION in some men who have used the medicine. While the FDA has concluded that no causal relationship between Viagra and NAION has been established, Pfizer has agreed to include on the Viagra label that these reports exist in order to inform physicians and patients.

Pfizer said it is the company's understanding that the FDA has asked that similar information be included in the labels of all oral ED medicines.

Shares of the company fell 44 cents to close earlier at $28.08 on the New York Stock Exchange.