PHILADELPHIA — Stents and coronary artery bypass surgery are no more effective than intensive drug treatment and better health habits in preventing millions of Americans from heart attacks and death, a large study found, shedding new light on a major controversy in cardiology.

Researchers and doctors have fiercely debated for years how best to treat people who have narrowed coronary arteries but aren’t suffering acute symptoms.

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The standard treatment has been to implant stents—wire mesh tubes that open up clogged arteries—or to perform bypass surgery, redirecting blood around a blockage. Those procedures are performed even though these patients either have no symptoms or feel chest pain only when they climb a few flights of stairs or exert themselves in some other way.

The study is the largest and among the most rigorous research yet to suggest that while stents and bypass surgery can be lifesaving for people who are having heart attacks, they aren’t necessarily better than cholesterol-lowering drugs and other changes in health habits for most people with chronic, or stable, coronary artery disease, which affects about 9.4 million Americans.

“You won’t prolong life,” said Judith Hochman, chair of the study and senior associate dean for clinical sciences at the New York University Grossman School of Medicine.

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But stents or bypass surgery work better than medicine and lifestyle changes alone in relieving symptoms for people who have frequent angina, or chest pain, the researchers found.

The findings, released Saturday at the American Heart Association’s annual scientific conference, should prompt more discussion between patients and their doctors about treatment, she said. “Statins and aspirin are critically important,” she said. “We need to understand better how to get people to modify their risk factors.” Lifestyle changes can be hard to make and sustain, she said.

This story continues in The Wall Street Journal.