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Anticoagulant medicines

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Anticoagulant drugs are used to reduce the ability of the blood to clot. Examples of anticoagulants include aspirin, heparin and warfarin.
Brought to you by NHS Choices



In a healthy person, the body is able to protect itself from excessive bleeding by allowing a part of the blood called plasma to stick together and form clots. These clots are formed at the site of a wound or injury where blood might otherwise leak out of the body or into the wrong internal organs.

Clotting is a natural mechanism that relies on a number of chemical reactions occurring within the body in order to produce a substance called thrombin. This substance then acts upon a protein called fibrinogen, and converts it into fibrin. Fibrin creates a number of threads that make the plasma sticky, enabling a clot to form.

The process of forming clots is therefore complicated and relies upon a lot of processes happening correctly. This means that things can go wrong if one or more parts of the process fail to work. Sometimes the blood does not clot easily and there is a risk of excessive bleeding (haemorrhage). Sometimes it clots too much, putting the patient at risk of complications such as strokes or heart attacks.

Anticoagulant drugs are used to reduce the ability of the blood to clot. Examples of anticoagulants include aspirin, heparin and warfarin.

How it works

How anticoagulants work

Warfarin is a commonly prescribed anticoagulant drug, taken in tablet form, to slow the time of clotting in the blood. It interferes with the body's natural chemical process of clotting, by targeting an essential substance called vitamin K.

Heparin occurs naturally in the blood. It is also used as a drug, through injections or a drip into a vein. This has an effect on the substance thrombin, which is part of the chemical process that causes clotting. By increasing heparin levels in the blood, the time taken for a clot to form is increased. It is most often used for treating deep vein thrombosis and pulmonary embolism.

A combination of different anticoagulant drugs may be used.


Although some anticoagulants such as aspirin are available freely without prescription, they should always be taken following the instructions of your GP or other healthcare specialist. Taking too much of these drugs can result in severe bleeding, especially if you are bruised or injured. Aspirin should not be taken by children under 16, owing to its links with Reyes Syndrome.

These drugs often interact with other substances so your GP should be informed of any other medications or supplements you are taking.

If a person who is taking anticoagulants is going to undergo surgery, then they should make sure the medical team is informed of their medication. It may be necessary to stop taking these drugs in order to prevent excessive bleeding during surgery, and to promote healing.

There is a risk that taking anticoagulants, while reducing the risk of strokes and other conditions caused by clotting, can increase the chances of a patient having a haemorrhage. Therefore your medication levels and possible risks should be carefully monitored.

What it is used for

What anticoagulants are used for

Anticoagulants are used when a person's blood is clotting too quickly. These drugs reduce the rate at which the blood clots, so that it is taken down to the correct international normalised ratio (INR) - this is the correct time that the patient's blood should take to clot. The normal range for INR is 0.9-1.2.

Reducing the blood's ability to clot is a useful tool in preventing conditions such as strokes (sometimes caused when a blood clot travels to the brain) and heart attacks. Anticoagulants are given to people who have a history of these conditions or who are at risk of developing them.

Other high risk groups include people who have:

  • deep vein thrombosis (DVT)
  • a pulmonary embolism (a blood clot in the blood vessels around the lungs)
  • angina.

It also includes those who have recently undergone heart surgery, such as valve replacement.

Side effects

A side effect common to all anticoagulants is the risk of excessive bleeding (haemorrhages), due to the blood being thinned. This is why people taking anticoagulants should be closely monitored to check that they are on the correct dosage of their drug. The most common test is the INR.

Warfarin can interfere with other medications and also with alcohol. It may not be effective if too much vitamin K is taken by the patient (through food or supplements) and it is not usually taken in pregnancy because there are increased risks of foetal abnormalities.

Long term use of aspirin is associated with damage to the stomach lining, which may lead to internal bleeding in the digestive system. Aspirin should only be taken long-term on the advice of a doctor, and any possible side effects should be monitored by your GP.

Although it occurs naturally within the body, extra amounts of heparin can have a number of side effects including hair loss and osteoporosis.

Useful links

NHS Choices links

External links

This article was originally published by NHS Choices

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