“If you’ve had a heart attack or stroke, there’s no doubt that taking low-dose aspirin is beneficial,” says Erin Michos, M.D., M.H.S., associate director of preventive cardiology for the Ciccarone Center for the Prevention of Heart Disease. “But if you don’t have heart disease, should you take it just in case? The answer for most individuals is probably not.”

Aspirin’s Proven Benefit. In addition to relieving pain, lowering fever and reducing inflammation, aspirin can prevent blood clots from forming. Blood clots, the leading cause of heart attacks and strokes, form when a plaque (cholesterol and other substances deposited on artery walls) ruptures and your body tries to contain the damage by creating a clot. When arteries are already narrowed by the buildup of plaque, a clot can block a blood vessel and stop the flow of blood to the brain or heart.

Taking a regular dose of aspirin diminishes the ability of your blood to clump together into clots by targeting the body’s smallest blood cells. Called platelets, they bind together when they encounter damaged blood vessels. While aspirin’s “blood thinning” quality can prevent heart attacks and strokes, it also can put you at higher risk for other harmful events.

Risks of Low-Dose Aspirin. Like most medicines, aspirin has side effects. It irritates your stomach lining and can trigger gastrointestinal upset, ulcers and bleeding. And, because it thins your blood, it can be dangerous for people who are at higher risk of bleeding.

Factors that make preventive use of aspirin dangerous include:

• Use of other medications that thin the blood
• A history of gastrointestinal ulcers, bleeding or gastritis
• Kidney failure or severe liver disease
• Bleeding or clotting disorders

Is there more harm than benefit? Previous guidelines from the United States Preventive Services Task Force warned against taking aspirin for the primary prevention of heart disease unless you’re at an elevated risk — typically if you’re 50 to 69 years old with a 10 percent or greater chance of having a heart attack or stroke within the next 10 years.

There is good reason to be wary of aspirin, warns Michos, particularly for women. The Women’s Health Study was a large trial that looked at whether women with no history of heart disease would benefit from taking a low dose of aspirin. Researchers found that in the overall group of women, aspirin didn’t reduce the risk of heart attacks, but it did increase the risk of bleeding. Some benefit was seen for women over the age of 65.

“So not only was there lack of benefit for the younger women taking aspirin, but there was also a question of harm,” says Michos. “It’s important for people to realize that just because aspirin is over-the-counter does not mean it is necessarily safe. Many patients take aspirin because they think it’s good for their hearts, but it carries some serious risks. ”

More recently, two large clinical trials comparing aspirin with placebo among people without known heart disease give even more reason to be cautious with an aspirin. The ARRIVE trial included men over the age of 55 and women over age 60 who were deemed to be at elevated risk for heart disease by having several risk factors. The ASPREE trial enrolled older adults (70 and older; African-Americans and Hispanics 65 and older). Both trials showed that low dose aspirin (at 100 milligrams per day) did not prevent subsequent heart attacks or strokes over a period of approximately five years. However, aspirin did increase the risk for major bleeding. Furthermore, in the ASPREE trial, there were more deaths attributed to aspirin use. Michos finds the new results “alarming” and says that most adults without known heart disease should not take aspirin routinely for heart attack and stroke prevention.

“I still recommend aspirin for those with known heart disease or stroke, or for select individuals who might be at particularly high risk due to evidence of significant plaque in their arteries, if they are not at high bleeding risk,” Michos says. “But for the rest of my patients at lower or intermediate risk, it seems that the risks of aspirin outweigh the benefits. Particularly for elderly patients, if they don’t have known heart disease, I would think carefully about using it. It is likely that other therapies such as the appropriate use of statin medications, more intensive blood pressure control and smoking cessation are more important measures for prevention than taking aspirin.”

The best way to assess your risk level is to talk to your doctor about it. Your doctor can help you weigh the risks and benefits to determine if low dose aspirin therapy is right for you.