An aspirin a day may do more harm than good

Published October 11, 2023

6 min read

Millions of people take aspirin every day to prevent a heart attack or stroke. But for some people, taking the drug might be risky.

Doctors have prescribed aspirin for decades, primarily to patients who already have a history of heart and blood vessel disease—the most common cause of death worldwide­­. Physicians also routinely prescribed daily doses of aspirin to patients without cardiovascular disease—a course of action that until recently was endorsed by the United States Preventive Services Task Force, an independent panel of experts who make recommendations about preventive medicines.

Aspirin remains a critical line of defense against a second heart attack. But last year, the U.S. Preventive Services Task Force changed its recommendations after two large clinical studies showed that as a preventive measure, aspirin’s risks likely outweigh its benefits for people ages 60 to 69—even if they’re at an increased risk of having a heart attack or stroke.

(Why heart attacks are rising in young adults—and what to watch for.)

Still, millions of people continue to use the drug. In the U.S. alone, a 2019 study found that about one in five people over 40 take aspirin every day. Many of those started taking the drug without a doctor’s recommendation—which experts warn could put them at greater risk of bleeding in the brain and gut. Meanwhile, millions of others who do need it are not taking it.

So how do you sort through all this changing guidance? Here’s what the experts say.

How does aspirin protect against heart attacks?

In the early 1900s, cardiologist Lawrence Craven noticed that his patients taking aspirin had fewer heart attacks. Since then, several clinical studies have shown that an aspirin a day keeps heart attacks away. A landmark 1989 study on more than 22,000 physicians without heart disease or a history of stroke showed that participants taking aspirin had 44 percent fewer heart attacks.

Aspirin, formally called acetylsalicylic acid, prevents cells called platelets from coming together to form clots. In patients with heart disease, platelets can clump up abnormally, cutting off oxygen to tissues and causing a heart attack or stroke. Aspirin thins the blood, effectively by making clot-forming cells called platelets less sticky. That’s because drug blocks an enzyme called cyclooxygenase, which sits on the surface of platelets, preventing it from kickstarting the production of hormone-like molecules called prostaglandins that are responsible for telling platelets to come together during injury.

(Are you taking the wrong medications? You might be surprised.)

Aspirin’s blood thinning properties, however, also make its users more likely to experience severe bleeding. Patients at a risk of bleeding shouldn’t take the drug, experts agree, and bleeding risk increases as patients get older.

Why isn’t daily aspirin good for everyone?

For people ages 40 to 59, the U.S. Preventive Services Task Force recommends that anyone with an increased risk of heart attack work with their doctors to assess the risks and benefits of starting daily aspirin. But after age 60, the panel recommends that patients not take aspirin as a primary preventive measure.

“This is a recommendation about starting aspirin in people who have no evidence of cardiovascular disease,” stresses Michael Barry, a professor of medicine at Harvard Medical School and the chair of the U.S. Preventive Services Task Force. “People who have had a heart attack or stroke are recommended to take the drug.”

The task force changed its recommendations in 2022 after two large studies showed that while the drug decreased the risk of heart attack or stroke, it also increased patients’ risk of major bleeding events, such as brain bleeds and bleeding in the gut.

(How you should dispose of unused or expired medications.)

These risks essentially canceled out the drug’s potential benefits for those 60 years and older who are at a higher risk of life-threatening bleeding than younger groups. The drug was still a small net benefit for the 40 to 59 age group, however.

How do you weigh the risks and benefits?

But weighing risks and benefits is not always so simple. Randall Stafford, a professor at Stanford School of Medicine, says that “there’s bound to be a group that’s kind of in between where there’s more ambiguity.” For people over 60 who haven’t had a stroke or heart attack but at are a high risk of having one in the future, he says, “there might be a population that should be on aspirin.” More research is required to determine who exactly is in this group.

The new studies also contradicted decades of evidence showing that aspirin had net benefits in adults over 60. Eduardo Hernandez, the president of the Texas Heart Institute and a professor of medicine at Baylor College of Medicine, says that might be because other preventative measures are now providing the same benefits as daily aspirin. More patients use medications to reduce blood pressure, like statins, as well as drugs to manage diabetes. Both types of medication have “come a long way,” Hernandez says.

(Do generic drugs work as well as brand name? Here’s what to know.)

Then there’s the murky relationship between aspirin and colorectal cancer. While aspirin has been found to prevent colorectal cancer, some studies also show that the drug might increase the risk of other cancers. That means doctors typically only prescribe the drug for people with a very high risk of developing colon cancer. “It’s a controversial area, and one that’s not addressed by our most recent recommendations,” Barry says.

So what do I do now?

The U.S. Preventive Services Task Force’s recommendations don’t address stopping low-dose aspirin for patients who are already on it. Barry stresses that patients who want to wean off the drug should talk to their doctor: “People should talk to their doctors if they’d like to reassess the benefits and harms of aspirin for them.”

On the flip side, experts say that some people with a history of cardiovascular disease might consider talking to their doctor about starting to take aspirin. Recent studies have shown that in high-income countries like the U.S., roughly 35 percent of individuals who would benefit from taking aspirin don’t do so. “We don’t have consistent care in that population,” Stafford explains. “Some people are slipping through the cracks.”

Ultimately, says Hernandez, “lifestyle changes have a really big impact on patient outcomes.” Those hoping to prevent heart attacks should eat healthy and exercise regularly, he stresses. “That’s as valuable or more valuable than aspirin therapy.”

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