Sydney Wolfe of the Public Citizen's Health Research Group said yesterday the instructions for using Viagra fail to adequately warn about the possible effects of the drug on patients who have certain heart or cardiovascular conditions.

``Viagra affects blood vessels in many parts of the body,'' said Wolfe. ``These effects are very worrisome.''

He petitioned the Food and Drug Administration to immediately convene an advisory committee to consider the risks associated with Viagra. There have been at least 39 deaths among American Viagra drug users, but the cause of death is not known for 13 patients, and key medical information is missing for some others.

Brad Stone, a spokesman for the FDA, said that the agency would consider the proposal submitted by Wolfe, but added, ``with the data now available, we still believe that Viagra is safe when used according to the labeling.''

Pfizer Inc., the New York maker of Viagra, said in a statement that the drug ``is safe and effective when used'' properly. The company said 3.5 million prescriptions, or 30 million tablets, have been dispensed in the four months since the drug was approved.

Wolfe said Viagra was approved as part of a ``dangerously rushed'' process that carried the drug from submission to FDA approval in just six months and without consideration by an FDA advisory panel.

The FDA-required labeling warns that Viagra should not be used by heart patients who are taking nitroglycerin or other nitrate drugs. The combination can cause a sudden, and possibly lethal, drop in blood pressure.

But Wolfe said ``we have concerns for three groups of patients who are not included in the label warnings.'' The American College of Cardiology and the American Heart Association on August 10 also cautioned doctors to be careful about prescribing Viagra for these patient groups.

The three groups are patients with: Reduced blood flow to the heart, but who are not being treated with nitrates. If such patients take Viagra and then experience a heart attack, treatment with nitrates could be lethal.

The Viagra instructions also warn about possible side effects among patients who are taking two drugs, erythromycin or cimetidine, or who have liver or kidney disease. All of these can cause Viagra to stay in the body longer.

Adolph Hutter Jr.. of the Massachusetts General Hospital, chairman of an American College of Cardiology panel, said that it is clear that ``the combination nitrates and Viagra is deadly'', but it has been less widely known that other heart conditions can be affected by Viagra.

``It can be dangerous for some patients to take Viagra even if they are not on nitrates,'' he said.

Hutter said that the ACC is not calling on the FDA to make changes in its approval of Viagra, but that doctors ``need to exercise caution'' in prescribing the drug for patients in those three groups.

``We felt the current regulation of Viagra by the FDA is adequate for the data that has accumulated,'' said Hutter.