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Aspirin to prevent blood clots


For people who have heart or blood vessel disease, taking low-dose aspirin each day can reduce the risk of heart attack and stroke.

The usual dose is one low-dose aspirin tablet (75 mg) each day.

If you ever suspect that a child has accidentally swallowed some aspirin, contact your doctor for advice straightaway.

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About aspirin to prevent blood clots

Type of medicine

An antiplatelet medicine

Used for

To prevent clots from forming in blood vessels

Also called

Acetylsalicylic acid

Available as

Tablets, gastro-resistant (enteric coated) tablets and soluble (dispersible) tablets

In your blood there are 'sticky' cells called platelets. When you cut yourself, the platelets stick to each other (clot) to seal the wound. Sometimes platelets stick to each other inside an artery - this is called a thrombus. A thrombus can block a blood vessel, and this is often the cause of a stroke or heart attack. This is more likely to happen if the walls of the arteries to your head or heart have areas which have become thickened with fat deposits. Aspirin reduces the stickiness of platelets, and this helps prevent the platelets from sticking to the inside of an artery and forming a thrombus. This reduces the risk of you having a heart attack or stroke. When aspirin is used in this way, it is often referred to as 'low-dose' aspirin. Each tablet contains 75 mg of aspirin. Low-dose aspirin can be recommended for people with heart or blood vessel disease, and for people who have had heart bypass surgery. Most people who have recently had a heart attack or stroke will also be advised to take daily low-dose aspirin to help to prevent it from happening again.

Low-dose aspirin tablets are available on prescription, and you can also buy them without a prescription. However, do not take regular low-dose aspirin without discussing the advantages and disadvantages of doing so with your doctor.

At higher doses, aspirin is used to relieve pain and high temperature (fever). There is more information about this use of aspirin in a leaflet called Aspirin for pain or fever.

Before taking low-dose aspirin

Some medicines are not suitable for people with certain conditions, and sometimes a medicine may only be used if extra care is taken. For these reasons, before you start taking aspirin it is important that your doctor or pharmacist knows:

  • If you are pregnant or breastfeeding.

  • If you have ever had a stomach ulcer.

  • If you have asthma.

  • If you are under 16 years of age.

  • If you have any problems with the way your liver works or with the way your kidneys work.

  • If you have a blood disorder such as haemophilia, or glucose-6-phosphate dehydrogenase (G6PD) deficiency.

  • If you have gout.

  • If you have high blood pressure (hypertension).

  • If you have ever had an unusual or allergic-type reaction after taking aspirin or a non-steroidal anti-inflammatory drug (NSAID). NSAIDs include ibuprofen, diclofenac, indometacin and naproxen. You should also let your doctor know if you have ever had an allergic reaction to any other medicine.

  • If you are taking any other medicines. This includes any medicines you are taking which are available to buy without a prescription, as well as herbal and complementary medicines.

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How to take low-dose aspirin

  • Before you start taking aspirin, read the manufacturer's printed information leaflet from inside the pack. It will give you more information about low-dose aspirin and will provide you with a full list of the side-effects which you may experience from taking it.

  • It is likely you will be advised to take one (75 mg) tablet of aspirin each day. Your dose may be different to this if, for example, you have recently had heart bypass surgery. Take the tablets exactly as your doctor tells you to.

  • You can take aspirin at whatever time of day you find easiest to remember, but try to take your doses at the same time of day each day. Most people prefer to take it in the morning with breakfast, as they find this helps them to remember to take it.

  • Take a look at the label on the packet to find out what type of aspirin tablet you have been supplied with. If you have been given dispersible aspirin tablets, take your dose stirred into a small amount of water. It is important that you take this type of aspirin tablet with something to eat. This helps to reduce the risk of any irritation to your stomach. Gastro-resistant aspirin tablets (also called enteric-coated, or EC) do not have to be taken with food, as these tablets have a special coating which helps to protect your stomach from irritation. These tablets must be swallowed whole without chewing, unless your doctor has told you otherwise. You can take them with a drink of water to help you swallow.

  • If you are taking gastro-resistant aspirin tablets, do not take indigestion remedies during the two hours before you take the aspirin or during the two hours after you have taken aspirin. This is because the antacid in the indigestion remedy affects the way the coating on this type of tablet works.

  • If you forget to take a dose, take it as soon as you remember. If you do not remember until the following day, skip the missed dose. Do not take two doses together to make up for a forgotten dose.

Getting the most from your treatment

  • Try to keep all your regular appointments with your doctor. This is so your doctor can check on your progress.

  • Before buying or taking any 'over-the-counter' medicines, check with your pharmacist whether the medicine is safe for you to take. You should not take any other medicines which contain aspirin, and you should also avoid buying any anti-inflammatory painkiller such as ibuprofen. Many cold and flu remedies contain aspirin or ibuprofen.

  • Aspirin should not be given to a child under the age of 16 unless it has been prescribed by a doctor to treat a specific condition. This is because it is possible that aspirin given to young children is associated with a condition known as Reye's syndrome. Reye's syndrome is a very rare disease that can be fatal.

  • If you are due to have an operation or dental treatment, tell the person carrying out the treatment that you are taking aspirin. This is because any bleeding may take longer than normal to stop.

  • If you ever suspect that a child might have taken aspirin accidentally, go to the accident and emergency department of your local hospital straightaway. This is very important because aspirin can cause serious problems when it is taken accidentally or in overdose. Take the container with you to show what has been taken, even if the pack is now empty.

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Can low-dose aspirin cause problems?

Along with their useful effects, most medicines can cause unwanted side-effects although not everyone experiences them. The table below contains some of the most common ones associated with aspirin. You will find a full list in the manufacturer's information leaflet supplied with your medicine. The unwanted effects often improve as your body adjusts to the new medicine, but speak with your doctor or pharmacist if any of the following continue or become troublesome.

Common aspirin side-effects

What can I do if I experience this?

Feeling sick (nausea), indigestion

Stick to simple foods, and if you are not doing so, take your dose of aspirin after a meal. If this continues, speak with your doctor for further advice

Increased risk of bleeding

If you notice any unexplained bleeding, speak with a doctor for advice

Important: aspirin may cause allergic reactions; this is more common in people who have asthma. Stop taking aspirin and speak with a doctor urgently if you have an allergic reaction or develop any breathing difficulties.

If you experience any other symptoms which you think may be due to the tablets, speak with your doctor or pharmacist for further advice.

How to store aspirin

  • Keep all medicines out of the reach and sight of children.

  • Store in a cool, dry place, away from direct heat and light.

Important information about all medicines

Important information about all medicines

Never take more than the prescribed dose. If you suspect that you or someone else might have taken an overdose of this medicine, go to the accident and emergency department of your local hospital. Take the container with you, even if it is empty.

This medicine is for you. Never give it to other people even if their condition appears to be the same as yours.

Do not keep out-of-date or unwanted medicines. Take them to your local pharmacy which will dispose of them for you.

If you have any questions about this medicine ask your pharmacist.

Report side effects to a medicine or vaccine

If you experience side effects, you can report them online through the Yellow Card website.

Further reading and references

Article history

The information on this page is written and peer reviewed by qualified clinicians.

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