FDA investigating blindness cases possibly linked to impotence drugs

U.S. regulators said Friday they are looking into possible cases of blindness among men using the impotence drugs Viagra, Cialis and Levitra

The U.S. Food and Drug Administration said it is still looking into the blindness cases, adding that it does not have proof that the drugs were responsible.

The FDA said it has 43 reports of blindness, including 38 men taking Viagra, four taking Cialis and one using Levitra.

• BACKGROUND: Faint Warning

The blindness is called non-arteritic anterior ischemic optic neuropathy, and can occur in men who have diabetes or heart disease.

Approved for use beginning in 1998, Viagra's listed common side effects include headache, upset stomach and facial flushing. Viagra has been taken by about 23 million men around the world.

In a statement, Pfizer defended the drug, pointing to a review of 103 Viagra clinical trials involving 13,000 patients that found no reports of visual loss.

"There is no evidence showing that [blindness] occurred more frequently in men taking Viagra than men of similar age and health who did not take Viagra," the statement said.

Shares of Pfizer ( NYSE:PFE) , the maker of Viagra, were off 55 cents US at $28.35 US on the NYSE.

Cialis is made by Eli Lilly & Co. Shares of Eli Lilly were down 5 cents at $59.05 US.