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Aspirin for prevention: A look at the potential benefits and risks

This is the first in a series of three blog posts on aspirin for prevention. It clarifies the potential benefits and harms of aspirin use.

Editor's note: In March 2019, new guidelines were released that recommend more limited use of aspirin for prevention of cardiovascular disease. This is the first installment of a revised blog series.

When my doctor first asked me to take aspirin, I wasn't so sure I needed it. Since the 1980s, aspirin has a proven record of preventing second heart attacks and strokes, but its use in people without these problems was and remains a source of confusion for both doctors and patients. Why take a medicine that can cause severe bleeding problems if it is not clear that you'll personally benefit?

As I encountered more patients with questions about aspirin, it eventually dawned on me that the key was to look at the chances it would be beneficial and the chances it would cause harm. For any patient (including myself), aspirin for prevention should be taken only if its benefits outweigh its risks.

Let's look at one patient who is similar to many I've seen in the clinic:

Fred is a 58-year-old sales manager with high cholesterol and high blood pressure. He has never had a prior heart attack or stroke, but he smokes a pack of cigarettes daily. While 15 pounds overweight, he eats a healthy, mostly plant-based diet and walks a half an hour during his lunch breaks at work. Fred takes atorvastatin for his cholesterol and lisinopril for hypertension. A friend tells him he should consider taking low-dose aspirin.

What should Fred do next?

First, Fred should understand that taking aspirin has many potential benefits and several important possible harms. For some people, especially those with known damage to their arteries, the benefits outweigh the harms. Everyone who has had a stroke or heart attack should be on aspirin (or an alternative, anti-clotting medication).

For patients without a history of heart disease or stroke, sometimes aspirin is not very helpful. Non-smoking, non-diabetic women under 60 and men under 50 seldom benefit from aspirin.

For others, it is not always clear whether aspirin is beneficial, and further discussion with a doctor is important. Even though aspirin is available over-the-counter without a prescription, Fred should schedule an appointment with his doctor to clarify the potential benefits and harms prior to starting aspirin.

Aspirin benefits

Aspirin interferes with small blood components known as platelets. By making platelets less sticky, aspirin is able to prevent blood clots, or clumps -- including those in the arteries that flow to the heart and the brain -- from forming. When clots occur in these critical blood vessels, it's bad news. In the brain, a clogged cerebral artery causes a stroke resulting in dead brain tissue. When in an artery in the heart (coronary), the resulting heart attack kills off heart muscle.

In men, aspirin mostly prevents heart attacks, while in women aspirin prevents strokes. For both men and women, aspirin prevents several cancers, especially colon and rectal cancer. People at greater risk of having heart attacks or strokes will benefit the most from aspirin. Some factors that magnify risk include older age, being male, smoking, diabetes, high cholesterol, and high blood pressure.

Aspirin harms

The same process that makes aspirin beneficial can lead to problems.

When aspirin makes platelets less sticky, they cannot do their normal job of stopping abnormal bleeding. This can result in excessive bleeding in unexpected locations within the body. In the stomach, this can lead to hospitalization and the need for blood transfusions. When it happens in the brain, this causes an unusual bleeding (or hemorrhagic) stroke. This type of bleeding is about 40% more common in those taking aspirin.

Some factors make people more likely to bleed on aspirin, including a history of stomach ulcers and older age. Common pain relievers like naproxen (brand name, Aleve) and ibuprofen (brand names, Motrin and Advil) can make bleeding more likely, as can blood thinners like clopidogrel (Plavix) or warfarin (Coumadin).

The key question with aspirin is: which is greater, the possible preventive benefits or the chance of excessive bleeding. People with multiple risk factors that increase their chance of having a heart attack or stroke (like diabetes) usually have more benefits than harms. They should consider taking aspirin.

Those at lower risk for heart attacks and strokes are more likely to be harmed by aspirin -- they should not take aspirin.

Patient preferences matter as well. If a person is willing to tolerate potential bleeding complications in exchange for a lower chance of a stroke or heart attack, then this should weigh in favor of using aspirin.

Fred's doctor tells him to come in for lab tests and asks that he record his blood pressure at home. Gathering information about his risk factors will be important when they meet to compare the possible benefits of aspirin with its possible harms.

Like Fred, an important first step in taking care of yourself is to find out your risk factors. Do you know your blood pressure, fasting blood glucose, total cholesterol, and HDL?

This is the first in a series of three blog posts on aspirin for prevention. The next post will show you how to calculate your risk of future serious heart or stroke events and how to interpret this critical information so that you're ready to meet with your health care provider.

Randall Stafford, MD, PhD, a professor of medicine and director of the Program on Prevention Outcomes and Practices, is a primary care internal medicine physician at Stanford. He is developing practical strategies to improve how physicians and consumers approach chronic disease treatment and prevention. Additional information about the preventive use of aspirin is available at

Photo by Ragesoss

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