The US Food and Drug Administration has confirmed that it investigating reports that up to 38 men who used the impotence drug Viagra have suffered vision loss.

Pfizer, the company that makes Viagra and the largest pharmaceuticals group in the world, defended its product, saying that there is no evidence that the type of vision loss reported in the Viagra users is more frequent in users of the drug than it is in men who do not use it.

The company also said that the type of vision loss being reported by users of the drug is common in men over 50 who have conditions like high blood pressure, high cholesterol, and diabetes, conditions that can also cause the impotence problems that Viagra treats.

This, Pfizer claims, makes it impossible to tell whether the drug itself had anything to do with the men’s loss of vision. Notwithstanding their defense of Viagra, Pfizer is in talks with the FDA over the possibility of revising safety warnings on the drug.

Shares of Pfizer lost nearly 2 percent of their value on the New York Stock Exchange Friday after reports of the vision problems were announced.