The U.S. Food and Drug Administration said Friday it is investigating reports of blindness in a small number of men who have taken impotence drugs, including Viagra.
Officials have not determined if the drugs caused the vision problems.
A drug company spokesman says the same medical problems that cause impotence can also put men at risk for certain kinds of vision problems.
Slightly more than 40 cases of blindness have been reported among the tens of millions of men around the world have used impotence medication.
The FDA says it is talking to manufacturers of these drugs, including Viagra, Cialis, and Levitra, about strengthening warning labels.
Pfizer is the world's largest drug manufacturer and it markets Viagra, the best-selling impotence medication. The company sold nearly $1.7 billion worth of the drug last year.
Some information for this report provided by AP, Bloomberg and Reuters.