Giving aspirin to heart patients soon after bypass surgery dramatically lowers the risk of death and complications, contradicting long-held worries that the tablets can trigger disastrous bleeding, a major study found.

The surprising results – from 5,065 patients at 70 medical centers in 17 countries – should quickly change how doctors handle the 1 million patients worldwide who undergo coronary bypass surgery each year, said researchers at the Ischemia Research & Education Foundation and other experts.

Foundation researchers estimate giving a 5-cent aspirin within hours of bypass surgery could prevent about 27,000 deaths and 51,000 serious complications annually worldwide. That would also save billions of dollars, given the lower complication rate and shorter hospital stays.

“The results are so strikingly positive and so definitive,” said Dr. Robert Bonow, president of the American Heart Association. “It’s studies like this that actually change practice.”

Aspirin has been a mainstay of treating and preventing heart disease for a generation because it thins the blood and prevents clots. Yet many doctors are reluctant to give it soon after or shortly before bypass, fearing it will interfere with clotting and cause life-threatening internal bleeding.

In the study, about 60 percent of the patients received aspirin in doses up to 650 milligrams. Those getting aspirin within 48 hours of bypass surgery were only one-third as likely to die in the hospital as the others.

The aspirin group also was only half as likely to suffer a heart attack or stroke while still in the hospital, only one-fourth as likely to suffer kidney failure and about one-third as likely to have bowel damage from blood clots.

“If my mother were going into surgery now, there’s no question she would get aspirin postoperatively,” said the lead researcher, Dr. Dennis Mangano, founder of the Ischemia foundation, a nonprofit research group in San Francisco.

The study, conducted from 1996 through 2001, was reported in Thursday’s New England Journal of Medicine.

The heart association and the American College of Cardiology in 1999 recommended giving patients 100 mg to 325 mg of aspirin within 24 hours of bypasses using grafts from leg veins. The guidelines do not cover grafts using chest arteries or both veins and arteries, but Mangano said that in the study, aspirin benefited patients getting every graft type.

In another surprise, the study also found patients who stopped taking aspirin before surgery were more likely to die than those who kept on taking it. That, too, goes against the conventional wisdom; in fact, American Heart Association guidelines call for taking many heart patients off aspirin 7 to 10 days before bypass surgery.

Dr. Daniel Shindler, associate professor of medicine and anesthesiology at Robert Wood Johnson Medical School in New Brunswick, N.J., said the new study could change that practice, too.