The millions of people who take aspirin every day to prevent heart attacks probably need to have serious conversations with their doctors about whether they truly need it.

About 29 million people 40 and older took an aspirin a day in 2017 despite not having heart disease, according to a study by Harvard University and Beth Israel Deaconess Medical Center that was published Monday.

The study found about 6.6 million of those people used aspirin even though a doctor never recommended it to them. Nearly 10 million people older than 70 who didn’t have heart disease took daily aspirin for prevention, the researchers reported in Annals of Internal Medicine.

Multiple, extensive studies last year found that only a marginal benefit, if any, could be found from routine aspirin use – especially among older adults.

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A study published this year in the journal JAMA Neurology found that taking low-dose aspirin is associated with an increased risk for bleeding within the skull for people without heart disease.

The studies run counter to what doctors had recommended for decades: taking 75 to 100 milligrams of aspirin daily to prevent strokes or heart attacks.

“Many patients are confused about this,” said Colin O’Brien, a senior intern medicine resident at Beth Israel who led the most recent study.

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The studies prompted the American Heart Association and American College of Cardiology to change their guidelines in March:

People over 70 who don't have heart disease – or are younger but at increased risk of bleeding – should avoid daily aspirin for prevention.

Only certain 40- to 70-year-olds who don't have heart disease are at high enough risk to warrant 75 to 100 milligrams of aspirin daily, and that's for a doctor to decide.

The Harvard study shows how many millions of people who were taking a routine aspirin in 2017 should take a second look at the guidelines.

“Clinicians should be very selective in prescribing aspirin for people without known cardiovascular disease,” cardiologist Roger Blumenthal, who was not involved in the Harvard study, said in a statement in March. "It’s much more important to optimize lifestyle habits and control blood pressure and cholesterol as opposed to recommending aspirin."

Although people without a history of heart problems shouldn’t take routine aspirin, it’s still recommended for heart attack survivors.

The American Heart Association and American College of Cardiology say exercising regularly, maintaining a healthy weight, avoiding tobacco and eating a diet rich in vegetables and low in sugar and trans fats are among the best ways to prevent cardiovascular disease.

"We hope that more primary care doctors will talk to their patients about aspirin use, and more patients will raise this with their doctors," O'Brien said.

Contributing: Ashley May, USA TODAY, and The Associated Press. Follow Adrianna Rodriguez on Twitter: @AdriannaUSAT