Harvard Heart Letter

Daily aspirin for disease prevention: When do the benefits outweigh the risks?

A daily aspirin may help defend against heart disease, but it should still be used with caution.

daily aspirin disease prevention
Image: FlairImages/Thinkstock

Aspirin tablets have been a staple of home medicine cabinets and first-aid kits for nearly 100 years. Long before that, people chewed willow tree bark, which contains aspirin-like compounds, to treat a variety of ailments. On top of being an excellent painkiller and fever reducer at its standard dosage, aspirin dramatically reduces the risks for a second heart attack and certain types of stroke when taken daily at a low (81-mg) dose. Research also suggests that aspirin might help limit the growth of colorectal cancer and possibly inhibit other cancers as well, but more research in this area is needed.

These benefits, coupled with the fact that aspirin is both cheap and relatively safe, have led the U.S. Preventive Services Task Force (USPSTF) to recommend that even some healthy people take a daily aspirin to ward off future disease. But figuring out exactly who is likely to benefit most from this therapy is a more complex calculation, says Dr. Michael Gaziano, a professor of medicine at Harvard Medical School and cardiologist at Brigham and Women's Hospital.

Think it's a heart attack? Call 911, then chew an aspirin

If you think you're having a heart attack, doctors recommend chewing an aspirin, right after you call 911. Chewing the pill gets the anti-clotting chemicals into your bloodstream much faster than if you swallow it. If you don't have 325-mg pills, take four 81-mg pills. Failing that, most emergency medical services will give you one en route to the hospital or once you're there.

The secret to aspirin's success

Aspirin was the first of a class of medicines called nonsteroidal anti-inflammatory drugs (NSAIDs). These substances block key enzymes that cause inflammation and trigger the formation of blood clots. Since many heart attacks and strokes occur when a clot stops blood flow in a vessel that feeds the heart or brain, disrupting the clot-forming process lowers the odds of this happening. The anti-inflammatory effects of aspirin might even help with fighting cancer, though that remains unproved.

The flip side of aspirin's anti-clotting power is that blood takes longer to coagulate after a cut or injury. While some effects—such as more noticeable bruising—are minor, serious internal bleeding, especially in the brain, can be deadly. Aspirin also irritates the stomach lining, so gastrointestinal bleeding is a major concern for people who take aspirin regularly.

This is where the clinical decision making comes in. Once you've had a heart attack or stroke, the benefits of aspirin for preventing a repeat occurrence offset the risks of serious bleeding by about six to one. "My feeling is that most of the people who have bleeding events from aspirin don't die. But many people who have a heart attack or stroke do, so the choice is clear," says Dr. Gaziano.

The tipping point

It's a murkier decision for people who do not have a strong history of heart disease. For this group, the scales tip in favor of a daily aspirin only when a person has a 10% or greater risk of having a heart attack or stroke within the next 10 years. (You can find a variety of tools for this calculation at www.health.harvard.edu/147.)

In the interest of simplicity, the USPSTF guidelines decided to focus on age, a powerful determinant for both cardiovascular and cancer risk. According to their analysis, a daily low-dose aspirin might offer protection from both cardiovascular disease and colorectal cancer without causing undue bleeding in people ages 50 to 59. Because bleeding risk goes up with age, people ages 60 to 69 may get a smaller net benefit, but they might consider aspirin on a case-by-case basis. The evidence was too weak to make broad conclusions for people outside those age brackets.

What's right for you?

Although the chances of excess bleeding when taking low-dose aspirin are minimal in the general population, don't dismiss the risk too quickly, Dr. Gaziano cautions. People who take other clot-preventing medications may need to avoid aspirin. Those with recent gastrointestinal bleeding should likely avoid aspirin, as should some people who are facing major surgery. It should also be noted that the FDA does not endorse the use of daily aspirin in healthy people, citing the hazards of uncontrolled bleeding. "Ultimately, the starting point for making this determination needs to be a thoughtful conversation with your own doctor," says Dr. Gaziano.

The Aspirin Guide App

Researchers at Brigham and Women's Hospital developed a free app for Apple mobile devices (available on iTunes) to help guide the decision about daily aspirin use for disease prevention. Called Aspirin Guide, the app computes your risks of cardiovascular disease and bleeding. Those findings are then combined with your age, sex, and personal preferences to determine if you're a good candidate for aspirin therapy. The assessment takes only seconds and can be done during a routine doctor's visit. Afterward, a summary of the decision-making process can be emailed directly to you and logged in your medical record.