Many studies have looked at the risks and benefits of ASA. An analysis of 15 of these studies looked at 100,000 men and women without diagnosed heart disease. Their average age was 60 years. The analysis was explained in terms of 1000 people who take ASA, who have a moderate risk (10 to 19 per cent) of heart disease, and an average risk of cancer (12 per cent), over 10 years.

So, for every 1000 people, we can expect to see:

six fewer deaths
17 fewer non-fatal heart attacks
six fewer cancers
no significant reduction in non-fatal strokes
16 more major bleeding events (bleeding within the brain or digestive system that require hospitalization and/or blood transfusion).

For a certain group, taking ASA each day is clearly the right choice. People with established heart disease will benefit, especially those who have already had a heart attack. For these people, ASA has been shown to lower the chance of a second heart attack, stroke, and sudden death.

For those without established heart disease, the benefits of taking ASA are less clear. While ASA was proved to reduce the rate of heart attacks, it has not been shown to reduce the rate of stroke or death from cardiovascular disease. The major risk of taking ASA is its potential to cause bleeding. Bleeding is usually in the stomach and digestive system, but it can also occur in the brain and other parts of the body.

The greater your risk of heart disease, the more benefit you are likely to gain by taking ASA. The lower your risk of heart disease, the less benefit you will receive. For everyone, there is a risk of bleeding. If your risk of bleeding is greater than the protective benefit, then it does not make sense to take ASA.

How is cardiac risk determined?

Many risk calculators can produce a number to measure your risk of heart attack and stroke. Your doctor will use one to calculate your likelihood of having a heart attack or stroke in the next 10 years. Your age, gender, and blood pressure are all factors that contribute to this number. Whether or not you smoke or have diabetes, your cholesterol level, and family history also play a part. Your weight, activity level, and stress level will also be considered.

In Canada, the following guidelines are used for risk of heart attack and stroke.

Low: a risk less than 10 per cent is currently considered to be low. At this level of risk, ASA adds very little benefit. For instance, someone with a nine per cent risk who takes ASA only lowers the risk to eight per cent.
Moderate: between 10 and 19 per cent is a moderate risk. In this case, ASA is worth considering, especially if the bleeding risk is low.
High: above 20 per cent is high risk, and ASA is recommended.

For most people, the benefit gained by ASA in the highest risk group exceeds the chances of major bleeding. Daily ASA is recommended.

This decision becomes more difficult when someone has both a high risk of heart attack or stroke, and a high risk of bleeding. People with a history of ulcers, stomach or intestine bleeding, or regular use of non-steroidal anti-inflammatory medications must weigh this decision carefully.

How does ASA work?

Platelets are clotting agents that float around in the blood. ASA works by making them more slippery. Instead of the platelets getting stuck in narrowed arteries and creating blood clots, they slide more easily through the arteries. This enhances blood flow. ASA may achieve this effect by reducing inflammation inside the blood vessels.

Does ASA prevent cancer?

ASA has been shown in many studies to reduce the risk of certain cancers, most notably colon cancer. It appears that one must take ASA for several years – five or longer – to see this benefit. However, the best dose for this use is not well established, so the role of ASA in preventing cancer remains uncertain.

What do the current guidelines tell us?

When international guidelines are not consistent with one another, this shows that there probably is not universal agreement on the topic. This means that any benefit achieved by taking ASA is low enough to be controversial.

The current Canadian guideline suggests using ASA for prevention in people without known heart disease only if the risk is high enough. The therapy is not for everybody. Those with established heart disease should take ASA if their bleeding risk is not too high.
One current American guideline recommends a lower daily dose of ASA (81 milligrams) for anyone over age 50 without known heart disease. The guideline notes that the benefits are small. Another current American guideline recommends the use of ASA only in higher risk groups.

The current European guideline advises against the use of daily ASA for people without known heart disease.

Should I stop ASA before surgery?

Aspirin may increase the risk of bleeding during and after surgery. If you are having surgery, make sure you have clear instructions from your doctor about whether to stop taking aspirin before the surgery, when you should stop it, and when you should re-start it after the surgery.

How can I decide whether ASA is right for me?

The first step is to discuss using ASA with your doctor. If you have had a heart attack or stroke, or have known heart disease, you will likely benefit from taking it daily. If you do not have known heart disease, you must calculate your cardiac risk with your doctor. If you fall into the moderate- or high-risk category, and have a low bleeding risk, ASA might be the right choice for you. If you fall into a low risk category, there is little added benefit. A strong family history of certain types of cancers of the digestive system is another factor. In this case, it might also be recommended, although the benefits are small.

Remember, ASA is only one approach to reducing cardiac risk. Quitting smoking, getting more exercise, losing weight, managing high blood pressure, and managing diabetes are other important actions to pursue whether you take ASA or not.

Ask the Doctor - Should I take an Aspirin Every Day? Weigh the risks before deciding

What does the research tell us?