The message for the public is that healthy older people should not begin taking aspirin.

“If you don’t need it, don’t start it,” Dr. McNeil said.

But those who have already been using it regularly should not quit based on these findings, he said, recommending that they talk to their doctors first.

Dr. McNeil also emphasized that the new findings do not apply to people who have already had heart attacks or strokes, which usually involve blood clots. Those patients need aspirin, because it inhibits clotting.

The study, named Aspree, is important because it addresses the unanswered question of whether healthy older people should take aspirin, said Dr. Dr. Evan Hadley, director of the division of geriatrics and gerontology at the National Institute on Aging, which helped pay for the research. The National Cancer Institute, Monash University and the Australian government also paid. Bayer provided aspirin and placebos, but had no other role.

“For healthy older people, there’s still a good reason to talk to their doctors about what these findings mean for them individually,” Dr. Hadley said. “This is the average for a large group. A doctor can help sort out how it applies individually. It’s especially important for people already taking aspirin who are over 70. The study didn’t include many people who had been taking it, and doesn’t address the question of continuing versus stopping.”

The most widely used guidelines for using aspirin to prevent disease came out in 2016 from experts at the United States Preventive Services Task Force. They recommend the drug to prevent cardiovascular disease and colorectal cancer in many people aged 50 to 59 who have more than a 10 percent risk of having a heart attack or stroke during the next 10 years. (That risk, based on age, blood pressure, cholesterol and others factors, can be estimated with an online calculator from the American Heart Association and the American College of Cardiology.)

For people 60 to 69 with the same risk level, the guidelines say it should be an individual decision whether to take aspirin.