Viagra caused no harmful changes in blood flow in men with severe heart disease, a study financed by the drug's maker found.

The study in Thursday's New England Journal of Medicine added to evidence that Viagra alone does not increase the risk of heart attacks.

Many men who are impotent also have heart disease, because the risk factors are often the same: old age, diabetes, high blood pressure and high cholesterol.

Researchers at the University of Pennsylvania measured blood flow inside the coronary arteries of 14 men with severe narrowing of at least one artery. They found no adverse cardiovascular effects and a slight improvement in coronary blood flow reserve.

Viagra did lower the men's blood pressure slightly, as previously reported.

Viagra has been linked to an increased number of heart attack deaths, mostly in patients who use it while taking nitrates, because the combination can lead to a sudden, severe drop in blood pressure. Sex also can increase the risk of a heart attack.

The study was led by Dr. Howard Herrmann, of the University of Pennsylvania. It was supported by a grant from Pfizer, which makes Viagra.