Wide World of Sex- The joy of sex that Viagra and other impotence drugs have brought to millions has a rare and alarming potential side effect – having the men go blind.

The maker of Viagra said yesterday it may change the warning label on its popular impotence drug after reports that at least 38 men taking the pricey blue pill have gone blind.

Pfizer, the world’s largest drugmaker, noted there is no evidence that Viagra directly causes vision loss, but acknowledged the company is talking with U.S. regulators about adding a warning for the rare side effect.

The Food and Drug Administration is also investigating reports of blindness in four men on rival drug Cialis and one man on Levitra, agency spokeswoman Susan Cruzan said.

Many men who take such drugs may already be predisposed to blindness, doctors said, because the same illnesses that cause erectile dysfunction, like high blood pressure, high cholesterol and diabetes, also increase the risk of suffering what amounts to a stroke in the eye.

“What it may end up being is just another link between vascular disease and erectile dysfunction and not necessarily a cause and effect of the [drugs],” said Dr. James Underberg, a vascular disease expert at NYU Medical Center in Manhattan.

That more men on Viagra suffered blindness may simply reflect the drug’s dominance in the market and the fact it has been around longest, he said.

Worldwide, 23 million men have used Viagra since it went on sale in 1998. It popularized the once-obscure medical term erectile dysfunction after a series of celebrity endorsements, including from former Republican Sen. Bob Dole and Texas Rangers slugger Rafael Palmeiro.

Cialis and Levitra – both launched in 2003 – have steadily chipped away at Viagra’s dominance. They now account for about 20% and 12% of the market, respectively.

Pfizer spokesman Daniel Watts said the New York-based drugmaker has seen no similar cases of blindness in 103 clinical trials involving 13,000 patients.

Eli Lilly, which makes Cialis, recently agreed to update its warning label to include more serious vision problems, the FDA’s Cruzan said.

“The reason you would add something to the label even though you haven’t established a cause and effect is so that doctors and patients are aware so they can take it into consideration,” she said.

The current label for Cialis refers to vision problems, including seeing a blue tinge, as an uncommon side effect.

Viagra’s label cautions users about possible increased sensitivity to light or blurred vision.

A recent study by a University of Minnesota ophthalmologist reported seven patients in their 50s and 60s showed vision-loss symptoms within 36 hours of taking Viagra.

Viagra, Cialis and Levitra all work the same way – by increasing blood flow in the penis. But the drugs also improve blood flow elsewhere, which could actually reduce strokes.

The drugs also have much more common side effects that patients should watch out for.

Men who take drugs that contain nitrates – often found in medication to treat angina – should not take Viagra because of the risk of sudden, unsafe drops in blood pressure. Other side effects are headache, indigestion and muscle aches.

Less common side effects include priapism – erections that last more than four hours. Priapism can cause long-term damage to the penis if not treated promptly.