Sure alcohol causes a long list of ills, but a major study yields the best evidence yet that a daily glass or two is a highly effective way to prevent heart attacks by raising the body’s supply of the good cholesterol.

The researchers conclude that moderate alcohol consumption cuts heart-attack risk in half, largely because drinkers have about 15 percent higher levels than non-drinkers of high-density lipoprotein cholesterol. HDL prevents heart disease by cleansing the blood vessels of fatty buildups.

The belief has been growing for at least two decades that a little booze is good for the heart. Lately, the focus has been on wine, which seems to confer upon the French the ability to enjoy buttery sauces and other fatty foods and still maintain a low rate of heart disease.

Skeptics have demanded a logical explanation of how alcohol could cause such a powerful effect. Evidence of an effect on HDL was murky, because earlier studies were too small to show a clear connection.

“We think we have found the mechanism by which alcohol may protect against heart disease,” said Dr. J. Michael Gaziano of Brigham and Women’s Hospital in Boston.

The study by Gaziano and others, first publicized in May, was published in Thursday’s New England Journal of Medicine with a supportive editorial.

“There now seems little doubt that alcohol exerts a protective effect against coronary heart disease,” Drs. Gary D. Friedman and Arthur L. Klatsky of Kaiser Permanente Medical Care Program in Oakland, Calif., wrote in the editorial.

The study has important implications, since heart attacks are the nation’s biggest killer, taking 500,000 lives annually. But the research also sharpens a medical dilemma for doctors. Many are reluctant to suggest people drink for their health.

The principal reasons are the well-known hazards of alcohol abuse, including cirrhosis of the liver, high blood pressure and strokes. Alcohol is a major cause of car accidents. It damages fetuses in the womb. And, ironically, too much alcohol causes heart disease.

The researchers stopped short of recommending that teetotalers take up alcohol or that occasional drinkers increase their consumption to once a day.

“The benefits have to outweigh the risks,” Gaziano said. “This is something that patients should discuss with their doctors.”

Doctors may be especially reluctant to suggest drinking for people with diabetes or high blood pressure, since it could aggravate their conditions.

Even for otherwise healthy people, some reports suggests moderate alcohol may increase the risk of breast and colon cancer, although this possibility is less solid than the evidence that it wards off heart attacks.

In their study, the doctors questioned 340 recent heart attack victims at six suburban Boston hospitals about their drinking habits. They compared them to healthy people the same age and sex.

The study found that those who consumed one to three drinks daily had half the heart attack risk of people who never drank. Consuming more than three drinks a day did not lower the risk further.

The risk was also reduced for those who drank at least once a month but not as often as once a day. But the benefit was smaller — about a 17 percent reduction.