WASHINGTON -- Viagra is now a decade old and is used by an estimated 35 million men to treat erectile dysfunction.

But now there are new warnings that men with ED may need to talk to their doctors before taking these pills because they could have a more serious problem: heart disease.

"We know men who have ED are almost 50 percent more likely to develop coronary artery disease," said urologist Dr. Edward Karpman.

The underlying risk factors for ED are similar to those for heart disease. They include smoking, obesity, hypertension and one of the most serious factors, diabetes. Two new studies indicate that diabetic men who have ED are more likely to suffer a serious heart attack.

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"Diabetics in general are at risk for heart disease," said Washington Hospital Center cardiologist Dr. Edward Morris. "So erectile dysfunction is simply, a significant independent risk factor for heart disease. So it adds more leeway to stress testing and so on to make sure they do not have heart disease."

Men typically show signs of ED more than three years before the onset of symptoms of heart disease. In the latest study, ED always occured in diabetic men before they developed heart problems. So Morris is warning that taking an impotence drug could be covering up one of the most obvious warning signs of heart disease.

"Erectile dysfunction puts you at three times higher risk for having heart disease, and therefore, if you're taking Viagra, which we know in the past can cause heart attacks, you should certainly be evaluated for heart disease."

The studies did show that taking cholesterol-lowering drugs like statins could lower the risk of heart disease for diabetics with ED. There was some evidence that Viagra could actually lower a man's risk for developing heart disease, but that link was pretty minimal.

Doctors stress that men should always tell their doctors if they're taking Viagra.