BOSTON -- Some men taking the impotence drugs Viagra, Cialis and Levitra have lost their sight, according to federal officials.

NewsCenter 5's Rhonda Mann reported that more than 23 million men take the impotence drug Viagra, and now the Food and Drug Administration is looking into whether it, along with similar drugs Levitra and Cialis, are linked to vision loss and even blindness.

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"In some cases, the patients lost visual acuity or eye chart vision, and in others, they lost peripheral vision. But in either situation, the loss of vision was permanent," said University of Minnesota ophthalmologist Howard Pomeranz.

According to the FDA, there have been 43 reports of varying degrees of vision loss -- 38 Viagra users, four Cialis users and one Levitra user.

The numbers could reflect Viagra's market share. About 70 percent of men who take one of these drugs take Viagra.

There are side effects with Viagra that include headaches, flushing of the skin, and vision problems, all of which are noted on the labeling.

But this condition is different, according to experts. Non-arteritic anterior ischemic optic neuropathy, or NAION, is caused by a change in blood flow to the optic nerve. Eye specialists said the condition is rare with less than 6,000 cases each year.

"The loss of vision occurs fairly suddenly, and over a period of a few days or week, the vision will decline further but then tends to stay at that lower level. There is relatively low opportunity for improvement," said Dr. Joseph Rizzo of the Massachusetts Eye and Ear Infirmary.

The condition is more common in those with diabetes and high blood pressure -- the same population most likely to use impotence medicines. And Pfizer, the maker of Viagra, pointed out that studies done on the drug show "no evidence" that "the condition occurred more frequently in men taking Viagra than men of similar age and health who did not."

Many doctors said they will continue prescribing the drugs without hesitation.

"We will have between 10 and 12 years of experience with this drug," said Dr. Ricardo Munarriz of the Boston University School of Medicine. "If this was a common side effect or associated with this condition, we would see it more frequently."

Pfizer said it is in discussions with the FDA to update their labeling to reflect the rare occurrences of this vision problem.