(CNN) -- Millions of US adults who've never had cardiovascular disease could still be taking a daily aspirin to prevent heart disease, despite updated recommendations that say it may be unnecessary and possibly risky, new research suggests.
The new research provides a snapshot of how many adults were estimated to be using aspirin for the primary prevention of cardiovascular disease in 2017 -- before the new guidelines came out in March of this year. The research didn't evaluate whether people continued to take daily aspirin after the new guidelines were released.
Nearly 30 million adults in the United States age 40 and older and without cardiovascular disease reported using aspirin to prevent the disease in 2017 -- and millions did so without a physician's recommendation, according to the research, published Monday in the journal Annals of Internal Medicine.
Among that same group, it's estimated that nearly half of the adults age 70 or older without cardiovascular disease reported using daily aspirin for the primary prevention of the disease, according to the research.
"What we were somewhat surprised at is the high numbers of older adults that were taking aspirin who don't actually have existing heart disease or stroke, and that's because it's been known that the older that you are, the higher the risk of bleeding from aspirin," said Dr. Christina Wee, an associate professor at Harvard Medical School and director of the obesity research program at Beth Israel Deaconess Medical Center in Boston, who was senior author of the new research.
"Our understanding of the benefits and harms of aspirin used to prevent heart disease is evolving and there have been recent changes in the evidence as well as a change in guidelines," she said. The general public "really should talk to their doctor to see whether they should or should not be on aspirin to prevent heart disease."
In March, the American College of Cardiology and the American Heart Association released new guidelines on the primary prevention of cardiovascular disease, indicating that a daily low-dose aspirin is no longer recommended as a preventative for older adults who don't have a high risk or existing heart disease.
Primary prevention refers to actions taken to avoid the initial manifestation of a disease, whereas secondary prevention refers to actions taken to reduce the impact of a disease or recurrence of an event.
The new guidelines are that low-dose aspirin "might be considered for the primary prevention" of cardiovascular disease among select adults aged 40 to 70 who are at higher risk of the disease but not at increased risk of bleeding, which can be a side effect of aspirin.
Low-dose aspirin should not be administered for the primary prevention of cardiovascular disease among adults of any age who are at increased risk of bleeding, according to the guidelines.
'A significant number of patients take aspirin on their own'
The research included self-reported data on aspirin use from the US Centers for Disease Control and Prevention's 2017 National Health Interview Survey, which asked adults 40 and older whether a doctor has ever told them to take a daily low-dose aspirin to prevent or control heart disease and whether they were following that advice, among other questions.
The researchers took a close look at that survey data, involving 14,328 adults who were nationally representative of the rest of the United States.
Since the data were representative of the rest of the nation, the researchers interpreted that would mean that, among adults age 40 and older without cardiovascular disease, 23.4% -- or about 29 million people nationwide -- would report taking daily aspirin for the prevention of cardiovascular disease, based on the survey responses.
Of those people, the data suggested that 22.8% -- or 6.6 million people -- would have done so without a physician's recommendation.
The study had some limitations, including that the data was based on self-reported survey responses and that adults younger than 40 were not asked about aspirin use.
In the survey data, "there is no way of knowing whether these patients should be on aspirin," said Dr. Sharon Reimold, cardiologist, professor, and vice chair of internal medicine at UT Southwestern Medical Center in Dallas, who was not involved in the new research.
"Not only do a significant number of patients take aspirin on their own, clinical changes from the newer recommendations are slow to catch on," Reimold said.
"Aspirin is frequently thought of as an over-the-counter medication and it may not appear on a patient's medication list," she said. "Patients need to remember to discuss this with their provider team and the provider team needs to remember to inquire about the use of over-the-counter therapies such as aspirin."
In response to the new research, Bayer Aspirin spokesperson Dr. Paul Gurbel said that a daily aspirin remains important for the secondary prevention of a heart attack or stroke.
"One of the concerns that I have when I see this kind of work being published in the literature is that it gets picked up by the lay press and that people who are on aspirin for secondary prevention stop their aspirin and that can be lethal," said Gurbel, director of interventional cardiology and cardiovascular medicine research at Inova Health System in Maryland, who was not involved in the new research.
"The risk would be, if they stop it, they could have a recurrent event," he said. "You don't want to ever take aspirin on your own, just like you should never take any medication on your own, unless you talk to a doctor first. And then you should never stop the aspirin or consider stopping it or changing the dosing regimen unless you talk to your physician first."
What a cardiologist is doing for her own patients
Since the updated guidelines were released, cardiologist Dr. Erin Michos said that she has been "actively de-prescribing" aspirin use among her own patients who are 70 and older if they do not have heart disease.
"Conversely, there has also been concern that some higher risk individuals with established heart disease might have misunderstood the new guideline recommendations and inappropriately stopped aspirin without discussing with their doctors first," said Michos, an associate professor and associate director of preventive cardiology at Johns Hopkins Medicine in Baltimore, who was not involved in the new research.
"It is important for the general public to know that these new recommendations about aspirin apply to healthy people who do not have pre-existing heart disease," she said. "In healthier people without pre-existing heart disease, the risks of aspirin -- namely bleeding from the gastrointestinal tract -- may outweigh the benefits, since the rates of vascular events are lower in this healthier population but bleeding risks are comparable."
The number of older adults estimated to still use aspirin in the new research, despite the risks, is "very concerning," Michos said.
Yet since the new research was based on 2017 data, before the new guidelines were released, more research is needed to determine whether that number has gone down.
Overall, she said, "patients should discuss with their doctors whether aspirin is right for them."