Aspirin to Prevent Blood Clots
And other antiplatelet drugs

Most people at increased risk of having a heart attack or stroke take a low dose aspirin (75 mg) each day. This reduces the risk of having a heart attack by about a third, and the risk of having a stroke by about a quarter. However, there is a small risk of serious side-effects with aspirin. In particular, bleeding from the gut. If you develop upper abdominal (tummy) pains, pass blood or black stools, or vomit blood, then stop taking the aspirin and see your doctor as soon as possible or go to the nearest casualty department.

What does aspirin do?

Aspirin is a drug that has been used for many years as a painkiller. However, it has another action to lower the risk of forming a blood clot in the arteries of the heart (coronary arteries) or brain arteries. This lowers the risk of having a heart attack or stroke.

How does aspirin work?

Aspirin helps to prevent blood clots forming. A blood clot may form in an artery (blood vessel) if a lot of platelets stick onto some atheroma. A clot in an artery may stop blood flowing to the tissues 'downstream'. If a blood clot forms in an artery in the heart or brain, it may cause a heart attack or stroke.

Atheroma patches are like fatty lumps that develop in the inside lining of some arteries. This mainly occurs in older people and is sometimes called 'hardening of the arteries'.

Platelets are tiny particles in the blood that help the blood to clot when a blood vessel is cut. Platelets sometimes stick onto atheroma inside an artery.

Low dose aspirin reduces the 'stickiness' of platelets. This helps to stop platelets sticking to a patch of atheroma and forming a blood clot.

What is the dose of aspirin to prevent blood clots?

The usual dose to prevent blood clots is 75 mg each day. This is a lot less than the dose for pain relief. Taking more than the recommended dose does not make aspirin work any better to prevent blood clots, but increases the risk of side-effects developing. Therefore, stick to the dose recommended by your doctor which is usually 75 mg daily. If you take low-dose aspirin to prevent blood clots and you need to take painkillers (for example, for headaches) it is best to take paracetamol rather than a higher dose of aspirin.

Who should take low dose aspirin?

People with known cardiovascular diseases
Cardiovascular diseases are diseases of the heart or blood vessels (vasculature). However, in practice, when doctors use the term 'cardiovascular disease' they usually mean diseases of the heart or blood vessels that are caused by atheroma. These include: heart attack, angina, stroke, TIA (transient ischaemic attack) and peripheral vascular disease. If you have or have had any of these diseases, you will normally be advised to take low dose aspirin to help to prevent further problems or complications.

People with a high risk of developing a cardiovascular disease
Everybody has some risk of developing atheroma that may cause one or more of the above cardiovascular diseases. However, certain 'risk factors' increase the risk. These include: high blood pressure, high cholesterol level, smoking, lack of exercise, obesity, an unhealthy diet, excess alcohol, a strong family history, certain ethnic groups, and being male. A separate leaflet called 'Preventing Cardiovascular Diseases' gives more details about risk factors.

Briefly, a 'risk factor calculator' is used by doctors and nurses to predict the health risk for an individual. A score is calculated which takes into account your risk factors (listed above). If you want to know your 'score', see your GP or practice nurse. Current guidelines advise that if your score gives you a 2 in 10 risk (or more) of developing a cardiovascular disease within the next 10 years, then treatment with low dose aspirin is advised if you are aged 50 or more.

If you have high blood pressure, you will normally only be prescribed aspirin once your blood pressure has been controlled by treatment.

People with diabetes
People with diabetes have an increased risk of developing cardiovascular diseases. If you have diabetes you will normally be advised to take low dose aspirin if you:

• Are aged 50 years or more, or
• Have had diabetes for 10 years or more, or
• Are taking treatment for high blood pressure, or
• Have any organ damage as a complication from diabetes.

Some people with atrial fibrillation
Atrial fibrillation (AF) is a condition of the heart. Briefly, people with AF have an increased risk of forming a blood clot which can cause a stroke. Some people with AF take warfarin to prevent blood clots forming and some take aspirin. See separate leaflet called 'Atrial Fibrillation' for details.

Why doesn't everyone take aspirin to prevent blood clots?

Because there is a small risk of developing serious side-effects (see below). In the groups listed above, the benefits far outweigh the small risk. However, for healthy people with a low health risk, even the small risk of side-effects from aspirin is not acceptable.

How beneficial is low dose aspirin?

For people with cardiovascular diseases, and those at high risk of developing cardiovascular diseases, there is great benefit from taking aspirin. Several studies involving thousands of people have proved that the risk of having a heart attack or stroke is much reduced in these people if they take aspirin. For example, the risk of having a non-fatal heart attack is reduced by by about a third. The risk of having a non-fatal stroke is reduced by about a quarter. The risk of dying is reduced by about a sixth.

Are there any side-effects from low dose aspirin?

Most people do not have any side-effects with low dose aspirin. The benefits of taking aspirin usually outweigh the small risk of developing side-effects. Read the leaflet that comes with the aspirin packet for a full list of possible side-effects.

The most serious possible side-effects that affect a small number of people include the following:

• Bleeding in the stomach or gut. This is more common if you have a stomach or duodenal ulcer. It is also more likely if you also take a steroid drug or an anti-inflammatory drug such as ibuprofen. As a rule, it is best to avoid taking both aspirin and these other drugs. If you develop upper abdominal pains, pass blood or black stools, or vomit blood, then stop taking the aspirin and see your doctor as soon as possible or go to the nearest casualty department.
• Rarely, some people are allergic to aspirin.
• Aspirin sometimes makes breathing symptoms worse if you have asthma.

If you have problems with taking aspirin then possible options include:

• Taking an alternative antiplatelet drug such as clopidogrel.
• If bleeding from the stomach or gut is a problem then another drug may be prescribed to 'protect' the lining of the stomach and gut.

Other antiplatelet drugs

Other drugs have a similar effect on reducing platelets from sticking together. They work in slightly different ways, acting on different chemicals, but with a similar end result of preventing blood clots. They include clopidogrel and dipyridamole.

As a rule, aspirin is usually the preferred drug. Sometimes an alternative is used if there is a problem with using aspirin. Sometimes, aspirin plus another antiplatelet drug are taken together. For example, after having a stroke a combination of low-dose aspirin plus dipyridamole is often prescribed for up to two years. In this situation, after two years the dipyridamole is stopped and the antiplatelet treatment is continued with low-dose aspirin alone.

In summary

Aspirin helps to prevent heart attacks and strokes. However, it is not a substitute for preventing atheroma from developing. If possible, you should also reduce any 'risk factors'. For example: do not smoke; do some regular physical activity; eat a healthy diet; keep your weight in check.

Also, if you are in a category where aspirin is advised, you are also likely to be advised to take medication to lower your cholesterol level and medication to lower blood pressure if it is high.

© EMIS and PIP 2006   Updated: October 2006   PRODIGY Validated