Aspirin and other antiplatelet medicines

What is aspirin?

Aspirin is a medicine 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. This lowers the risk of having a heart attack (myocardial infarction) or stroke.

Is aspirin a blood thinner?

Aspirin is sometimes called a blood thinner, because of this effect to reduce blood clots.

How does aspirin work?

Cross-section diagram of an artery with patches of atheroma

Aspirin helps to prevent blood clots forming. A blood clot may form in a blood vessel (artery) if a lot of platelets stick on to some atheroma (see below). A clot in an artery may stop blood flowing to the tissues further down. 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, which help the blood to clot when a blood vessel is cut. Platelets sometimes stick on to 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.

Who should take aspirin to prevent blood clots?

People with known cardiovascular diseases

Cardiovascular diseases are diseases of the heart or blood vessels. 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.

Patches of atheroma are like fatty lumps that develop in the inside lining of some blood vessels (arteries).

These diseases include:

• Heart attack.

• Angina.

• Stroke.

• Transient ischaemic attack (TIA).

• Peripheral arterial 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.

Taking aspirin when you have a cardiovascular disease, to reduce the risk of future cardiovascular diseases, is known as secondary prevention. For people with cardiovascular diseases there is a lot of 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 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.

Note: taking aspirin 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 and keep your weight in check.

What about 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.

• A high cholesterol level.

• Smoking.

• Lack of exercise.

• Obesity.

• An unhealthy diet.

• Excess alcohol.

• A strong family history of cardiovascular disease.

• Certain ethnic groups.

• Being male

See separate leaflet called Preventing cardiovascular diseases, which includes details about risk factors.

In the past, people at a high risk of developing a cardiovascular disease were recommended to take aspirin. This is called primary prevention. That is, aiming to prevent a disease occurring before it happens.

However, it is now known that for those who do not already have cardiovascular disease, the risks of taking aspirin outweigh any benefits.

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.

Are there any side-effects from aspirin?

Most people do not have any side-effects with low-dose aspirin.

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 take a steroid medicine or an anti-inflammatory medicine (such as ibuprofen, also known as an NSAID) as well. As a rule, it is best to avoid taking both aspirin and these other medicines. If you develop upper tummy (abdominal) pains, pass blood or black stools (faeces), or bring up (vomit) blood, stop taking the aspirin. Then see your doctor as soon as possible or go to the nearest casualty department.

• Rarely, some people are allergic to aspirin.

• Aspirin can occasionally make breathing symptoms worse if you have asthma.

If you have problems with taking aspirin to prevent blood clots, then possible options include:

• Taking an alternative antiplatelet medicine such as clopidogrel.

• If bleeding from the stomach or gut is a problem then another medicine may be prescribed to protect the lining of the stomach and gut.

Other antiplatelet medicines used to prevent blood clots

As mentioned earlier, platelets are tiny particles in the blood, which help the blood to clot. There are other medicines which have a similar effect on reducing platelets from sticking together. They work in slightly different ways, acting on different chemicals but with the similar end result of preventing blood clots.

They include:

• Clopidogrel.

• Prasugrel.

• Dipyridamole.

• Ticagrelor.

The choice of drug depends on your medical history:

• After an episode of acute coronary syndrome (ACS), dual anti-platelet therapy (DAPT) with aspirin and one other drug is recommended for one year, stepping down after that to just aspirin.

• For those with peripheral arterial disease (damage to the arteries in the legs), clopidogrel is the preferred anti-platelet.

• After a stroke or transient ischaemic attack, DAPT with clopidogrel or ticagrelor and aspirin is usually used for 3-4 weeks, then changing to either clopidogrel or ticagrelor.

• After an angioplasty (stent in the arteries around the heart), DAPT with aspirin and clopidogrel is used for between 1 month and 3 years, then stepping down to one drug.

Can aspirin prevent cancer?

The suggestion that aspirin use may prevent cancer was first made in 2010 and various studies have been done since then. In 2021, a large study meta-analysis (a type of paper which looks at lots of other papers) reviewed 118 studies. In those who had cancer and were taking daily aspirin, there was a reduction of about 20% in deaths from cancer and from other reasons, compared to those who weren't taking aspirin. Fatal bleeding complications were rare. In 2023, the same authors wrote a summary article in which they pointed out that the use of aspirin in patients with cancer would reduce blood clots and that some studies showed that aspirin reduces the likelihood of a cancer metastasising (spreading).

Current NHS guidelines recommend a daily dose of aspirin for people with Lynch syndrome, which increases the risk of several different types of cancer. It is not recommended on a population basis.

It is important to remember that the overall effect of taking aspirin is based on statistics of a community of people. It is impossible to say if you as an individual are likely to benefit, to be harmed, or for it to make no difference to you - just that the odds of you developing cancer go down, but you may develop a side-effect. The benefit has to be balanced against the risk. Aspirin is not a total preventer of blood clots or of cancer. It simply reduces the risk, and that risk varies from person to person.

Can you take aspirin with paracetamol?

Yes, this is fine. If you are ever in doubt as to whether you can take two over the counter medicines together, ask a pharmacist.