The U.S. Food and Drug Administration said Friday it was investigating reports of possible blindness by some users of the anti-impotence pills Viagra, Cialis and Levitra.

The FDA said it is still looking into the blindness cases, adding that it does not have proof that the drugs were responsible.




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

Also, a U.S. study warned this week that the recreational use of drugs such as Viagra by gay men is associated with higher risks of sexually-transmitted diseases, including HIV.

The type of blindness is called NAION, or non-arteritic anterior ischemic optic neuropathy. It can occur in men who are diabetic or have heart disease, the same conditions that can cause impotence and thus lead to the use of Viagra and similar medications.

Drug maker Pfizer said it was talking with the FDA about putting a warning label on Viagra bottles.

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But it stressed that reports of blindness were extremely rare among the 23 million men who had used the drug over the past seven years, and that NAION was no more common than among men who didn't use the drug.

Both erectile dysfunction and this type of blindness share underlying risk factors, such as high blood pressure, high cholesterol and diabetes and being more common among men over age 50, the drug maker said.

Viagra's listed common side effects include headache, upset stomach and facial flushing.

Gay men and drug abuse

A study published in the current issue of the American Journal of Medicine said Viagra-using gay men engaged in unprotected sex with partners of unknown HIV status up to six times as often as non-users

HIV-positive gay men were almost twice as likely to be diagnosed with an sexually transmitted disease if they were also Viagra users, and the newly HIV infected were 2.5 times as likely to be users, researchers from the San Francisco Department of Public Health found.

The study also found that more than half of these men were using other drugs such as methamphetamines or ecstasy at the same time, a fact which prompted an Arizona doctor's concern, expressed in an editorial, that drug abuse might make HIV more virulent.

"Could this have resulted in the development of the highly virulent HIV mutant?" wrote Dr. Joseph S. Alpert, at the University of Arizona Health Sciences Centre in Tucson.