A heart attack most often happens because blood flow through one or more of the coronary arteries is blocked. This blockage is usually caused by a blood clot that forms when plaque in the artery breaks open. A stroke occurs when a blood clot blocks a blood vessel in the brain (ischemic stroke). Aspirin can prevent blood clots from forming in your arteries. This may help prevent a heart attack or stroke.

People who have certain health problems shouldn't take aspirin. These include people who:

• Have recently had a stomach ulcer or bleeding in the digestive tract.
• Have recently had a stroke caused by bleeding in the brain.
• Are allergic to aspirin.
• Have asthma that is made worse by aspirin.

Aspirin causes your blood to clot more slowly. This increases your chance of bleeding problems, which can be serious.

Aspirin can cause:

• Bleeding in your stomach or another part of your digestive tract.
• Bleeding in other parts of the body, including the brain.

Most doctors think that a heart attack or stroke usually causes more damage than a serious bleed.

The risk of bleeding isn't the same for everyone. For example, people's risk of bleeding gets higher as they get older. Your doctor can help you know your risk of bleeding based on your age and your health.

Some people have other problems from aspirin. These include an allergic reaction, stomach pain, and nausea.

Your doctor can help you understand your risk of heart attack or stroke and your risk of bleeding. Then you can decide what's right for you.

• If you don't have a high risk of heart attack or stroke or if you have an increased risk of serious bleeding, the benefits of taking aspirin probably won't outweigh the chance of serious bleeding.
• If you have a high risk of heart attack or stroke and a low risk of serious bleeding, you may decide that reducing your risk of heart attack and stroke is worth the risk of bleeding. Or you may decide that the benefit isn't worth the risk.

You and your doctor may also talk about your age. People who are younger than 40 don't seem to benefit much from aspirin. For people older than 60 or 70, the risk of bleeding is likely greater than the possible benefit of taking aspirin.^{footnote 1, footnote 2} Your doctor can help you understand whether aspirin might help you based on your age.

Your doctor can estimate your risk for a heart attack or stroke in the next 10 years. Your doctor may use a tool called a risk calculator to figure out your risk. The tools are not perfect. They may show that your risk is higher or lower than it really is. But they can give you and your doctor a good idea about your risk.

There are different calculators, but they all include details about your health. These include:

• Your age, gender, and race or ethnic group.
• Your blood pressure and cholesterol numbers.
• If you have diabetes.
• If you're being treated for high blood pressure or high cholesterol.
• If you smoke or did in the past.

You can do other things to reduce your chance of having a heart attack or stroke. These things are important whether you take a daily aspirin or not.

• Eat healthy foods.
• Get regular exercise.
• Stay at a healthy weight. Lose weight if you need to.
• Don't smoke.
• Manage other health problems that increase your risk of heart attack and stroke. These include high blood pressure, diabetes, and high cholesterol.

Benefits

Evidence shows that for people with a high risk of heart attack or stroke, aspirin lowers the chance of heart attack, stroke, and dying from heart disease. The quality of this evidence is high.

Take a group of 100 people who have a high risk of heart attack or stroke. Here are their chances of having one in the next 10 years:^{footnote 5, footnote 3}

• Without aspirin, about 20 out of 100 people will have a heart attack or stroke or die from heart disease. This means that about 80 out of 100 won't have one.
• With aspirin, about 18 out of 100 people will have a heart attack or stroke or die from heart disease. This means that about 82 out of 100 won't.

Note that:

• This is just one example. Your risk of heart attack and stroke may be different.
• These estimates apply to people who are younger than 70 years old.^{footnote 4}

Risks

*These numbers are examples based on research studies.^{footnote 5, footnote 3}

Evidence shows that aspirin increases the chance of serious bleeding. The quality of this evidence is high.

Take a group of 100 people who have a low risk of bleeding. Here are their chances of having serious bleeding in the next 10 years:^{footnote 5, footnote 3}

• Without aspirin, about 1 out of 100 will have serious bleeding. This means that 99 out of 100 won't.
• With aspirin, about 2 out of 100 will have serious bleeding. This means that about 98 out of 100 won't.

Take a group of 100 people who have a moderate risk of bleeding. Here are their chances of having serious bleeding in the next 10 years:^{footnote 5, footnote 3}

• Without aspirin, about 10 out of 100 people will have serious bleeding. This means that 90 out of 100 won't.
• With aspirin, about 15 out of 100 people will have serious bleeding. This means that 85 out of 100 won't.

Take a group of 100 people who have a high risk of bleeding. Here are their chances of having serious bleeding in the next 10 years:^{footnote 5, footnote 3}

• Without aspirin, about 20 out of 100 people will have serious bleeding. This means that about 80 out of 100 won't.
• With aspirin, about 30 out of 100 people will have serious bleeding. This means that about 70 out of 100 won't.

Understanding the evidence

Some evidence is better than other evidence. Evidence comes from studies that look at how well treatments and tests work and how safe they are. For many reasons, some studies are more reliable than others. The better the evidence is—the higher its quality—the more we can trust it.

The information shown here is based on the best available evidence.^{footnote 5, footnote 3}

The evidence is rated using four quality levels: high, moderate, borderline, and inconclusive.

Another thing to understand is that the evidence can't predict what's going to happen in your case. When evidence tells us that 2 out of 100 people who have a certain test or treatment may have a certain result and that 98 out of 100 may not, there's no way to know if you will be one of the 2 or one of the 98.