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Anticoagulants are used to treat and prevent blood clots that may occur in your blood vessels. Warfarin is the most commonly used anticoagulant. However, newer anticoagulants are now available and are likely to be used more and more. If you take warfarin you need to have regular blood tests to monitor how quickly your blood clots. The main possible side-effect of anticoagulants is bleeding. Warfarin interacts with some other medicines and foods. If you are prescribed or buy any other medicine then tell your doctor, nurse or pharmacist that you are taking an anticoagulant.

Anticoagulants are medicines that prevent the blood from clotting as quickly or as effectively as normal. Some people call anticoagulants blood thinners. However, the blood is not actually made any thinner - it just does not clot so easily whilst you take an anticoagulant.

Anticoagulants are used to treat and prevent blood clots that may occur in your blood vessels. Blood clots can block blood vessels (an artery or a vein). A blocked artery stops blood and oxygen from getting to a part of your body (for example, to a part of the heart, brain or lungs). The tissue supplied by a blocked artery becomes damaged, or dies, and this results in serious problems such as a stroke or heart attack. A blood clot in a large vein, such as a clot in a leg vein (a deep vein thrombosis), can lead to serious problems. For example, it can lead to a clot that travels from the leg vein to the lungs (a pulmonary embolism).

A number of anticoagulants are available, including warfarin, acenocoumarol, phenindione, dabigatran, apixaban and rivaroxaban. All come in various different brand names. Warfarin, acenocoumarol and phenindione are older types of anticoagulants and have been used for many years in the UK. Dabigatran, apixaban and rivaroxaban are newer types of anticoagulant.

Aspirin also has an effect of preventing clots by preventing platelets sticking together. However, it is classed as an antiplatelet agent rather than an anticoagulant. It is not discussed further in this leaflet. See separate leaflet called Aspirin and Other Antiplatelet Medicines for more details.

Most people who need an anticoagulant are prescribed warfarin. However, the newer anticoagulant medicines (dabigatran, apixaban and rivaroxaban) are being increasingly used. Dabigatran, apixaban and rivaroxaban have a number of advantages over warfarin, although they still have similar kinds of side-effects.

Within seconds of cutting a blood vessel, the damaged tissue causes tiny cells in the blood (platelets) to become sticky and clump together around the cut. These activated platelets and the damaged tissue release chemicals which react with other chemicals and proteins in the blood, called clotting factors. There are 13 known clotting factors which are called by their Roman numbers - factor I to factor XIII. A complex cascade of chemical reactions involving these clotting factors quickly occurs next to a cut.

The final step of this cascade of chemical reactions is to convert factor I (also called fibrinogen - a soluble protein) into thin strands of a solid protein called fibrin. The strands of fibrin form a meshwork and trap blood cells and platelets which form into a solid clot.

If a blood clot forms within a blood vessel it can cause serious problems. So, there are also chemicals in the blood that prevent clots from forming and chemicals that dissolve clots. There is balance between forming and preventing clots. Normally, unless a blood vessel is damaged or cut, the balance tips in favour of preventing clots forming within blood vessels. However, sometimes a clot forms within a blood vessel that has not been injured or cut.

They interfere with chemicals needed to make clots or clotting factors.

Warfarin, acenocoumarol and phenindione block the effects of vitamin K which is needed to make some clotting factors described earlier. Blocking vitamin K prevents blood clots forming so easily by increasing the time it takes to make fibrin. It usually takes two or three days for these medicines to work fully.

Dabigatran, apixaban and rivaroxaban prevent a blood chemical called thrombin from working, which in turn prevents fibrin from being made from fibrinogen. Dabigatran binds to thrombin. Apixaban and rivaroxaban stop thrombin from being made. All three medicines work quickly - within two to four hours.

Anticoagulants are prescribed if you have already had a blood clot (treatment) - to prevent the clot from getting bigger, or if you are at risk of having a blood clot (prevention). Examples of people who are at risk of having a blood clot include anyone who has:

Warfarin acenocoumarol and phenindione

As discussed above, warfarin is currently the most prescribed anticoagulant. Acenocoumarol and phenindione are rarely used. They are mostly used when someone is unable to take warfarin - for example, if they are allergic to warfarin.


Until recently, dabigatran had not been prescribed as much as warfarin. However, it is thought that from now on this medicine will be prescribed more often. This is because it is now recommended as an option by national guidance for people with a certain type of atrial fibrillation. The National Institute for Health and Care Excellence (NICE) states that you should be able to have dabigatran etexilate as an option if you have a type of atrial fibrillation known as non-valvular and at least one of the following applies:

  • You have had a stroke, mini-stroke (transient ischaemic attack) or embolism.
  • Your heart pumps less blood around your body than normal.
  • You have symptoms of heart failure of New York Heart Association class 2 or above (mild shortness of breath and/or chest pain, and slight limitation during ordinary activity).
  • You are 75 or older.
  • You are 65 or older and have diabetes, coronary artery disease or high blood pressure.

Rivaroxaban and apixaban

Like dabigatran, until recently, rivaroxaban and apixaban had not been prescribed as much as warfarin. However, it is thought that from now on this medicine will be prescribed more often. This is because it too is now recommended as an option by national guidance for people with a certain type of atrial fibrillation. NICE states that you should be able to have rivaroxaban or apixaban as an option if you have a type of atrial fibrillation known as non-valvular and at least one of the following applies:

  • You have congestive heart failure (this occurs when the heart doesn't pump blood as well as it should).
  • You have high blood pressure.
  • You are 75 or older.
  • You have diabetes.
  • You have had a stroke or transient ischaemic attack at some point in the past.

There are several differences. The main one is that if you take warfarin, acenocoumarol or phenindione you need to have frequent blood tests to measure how quickly your blood clots. If you take dabigatran, apixaban or rivaroxaban this is not necessary. But you still need to have tests to make sure your kidneys are working well.

Warfarin, acenocoumarol and phenindione (unlike dabigatran, apixaban and rivaroxaban) all interact with lots of other medicines and foods. This means that after you take certain medicines or foods your blood may clot too quickly or too slowly.

Unlike warfarin, the dose of the newer anticoagulants usually does not change during treatment. There is usually no need to alter the dose.

The advantage of the older-type anticoagulants is that they have been used for decades, are known to work well and most of the possible side-effects are known. The newer types have been prescribed to far fewer people and have only been used for a few years. We are still learning about their possible side-effects.

Most people who take warfarin attend a warfarin clinic. This may be at your GP practice or at the local hospital. The clinic is run by a health professional specially trained in anticoagulation. He or she may be a doctor, specialist nurse, trained pharmacist, etc.

Blood tests

You will need regular blood tests to check on how quickly your blood clots when you are taking warfarin. Blood tests (and clinic visits) may be needed quite often at first but should reduce in frequency quite quickly. The aim is to get the dose of warfarin just right so your blood does not clot as easily as normal but not so much as to cause bleeding problems.

How to take warfarin

You will be advised on how to take warfarin and if it affects any other medication that you take. For example, the following are commonly advised:

  • You should aim to take warfarin at the same time each day. This is usually six o'clock in the evening.
  • If you accidentally miss a dose, NEVER take a double dose 'to catch up' (unless specifically advised by a doctor or by the person who runs the warfarin clinic).
  • Seek advice promptly if you think that you have taken too much warfarin by mistake or have missed any doses.

Other medication whilst taking warfarin

If you are prescribed or buy any other medicines then tell a doctor, nurse or pharmacist that you are on warfarin. This is because some medicines interfere with the way warfarin works and your dose may need to be altered. This is particularly important if you are using an anti-fungal preparation containing miconazole. Also, if you stop another medicine or change the dose, also seek advice, as your dose of warfarin may need to be altered. Some herbal medicines can also interfere with warfarin. For example, gingko biloba can increase the level of warfarin in the body, whereas ginseng can reduce the effect of warfarin.


If you have a major change in your diet or the foods that you eat then seek advice from the warfarin clinic. A major change in diet may mean that you need closer monitoring and may need a change in warfarin dose. Vitamin K, in particular, decreases the effect of warfarin. There are large amounts of vitamin K in certain foods such as liver, broccoli, Brussels sprouts and green leafy vegetables such as spinach, coriander and cabbage. Two other commonly eaten foods that are known to interact with warfarin are cranberry and grapefruit.

Women of childbearing age

You should seek advice promptly if you become pregnant or are planning a pregnancy. For safety reasons, warfarin is likely to be stopped and an alternative medicine called heparin is likely to be used instead.

This depends on what you are taking an anticoagulant for. Sometimes you only need to take it for a few weeks (after surgery) or months (DVT). On the other hand, some people need to take an anticoagulant for the rest of their lives (for example, people with atrial fibrillation or a mechanical heart valve). Your doctor will advise.

There are a number of possible side-effects with anticoagulants and it is not possible to list all of these here. However, the major side-effect of all anticoagulant medicines is bleeding. People who take warfarin, acenocoumarol and phenindione need to have regular blood tests to measure how quickly the blood clots. See the leaflet that comes with your particular brand for a full list of possible side-effects and cautions.

These medicines sometimes react with other medicines that you may take. So, make sure your doctor knows of any other medicines that you are taking, including ones that you have bought rather than been prescribed.

One indication that you may be taking too much anticoagulant is that you may bleed or bruise easily. Also, if you bleed, the bleeding may not stop as quickly as normally. If any of the following serious bleeding side-effects occur while you are taking an anticoagulant you should see a doctor urgently and have a blood test:

  • Passing blood in your urine or stools (faeces). Note: blood in faeces may be bright red. But, if you are bleeding from your stomach or small intestine, your faeces may turn a black or plum colour. This is called melaena. You should seek urgent medical attention if you have melaena.
  • Heavy bleeding during a period or other heavy vaginal bleeding (in women).
  • Severe bruising.
  • Prolonged nosebleeds (lasting for longer than 10 minutes).
  • Blood in your sick (vomit).
  • Coughing up blood.
  • If you cut yourself or have any other bleeding, you should seek medical help as soon as possible if the bleeding does not stop as quickly as you would expect.

Some other important things to consider are:

  • If you have any medical treatment you should always tell the healthcare professional looking after you that you are taking an anticoagulant. It is important that they know you may take longer to stop bleeding.
  • If you take warfarin you should always carry with you the yellow anticoagulant treatment booklet which will be given to you. This is in case of emergencies and a doctor needing to know that you are on warfarin, and at what dose.
  • If you have surgery or an invasive test then you may need temporarily to stop taking an anticoagulant.
  • Tell your dentist that you take an anticoagulant. Most dental work does not carry a risk of uncontrollable bleeding. However, for dental extractions and surgery you may need temporarily to stop taking an anticoagulant.
  • You should limit the amount of alcohol that you drink to a maximum of one or two units in any day and never binge drink.
  • Ideally, try to avoid activities that may cause abrasion, bruising, or cuts (for example, contact sports). Even gardening, sewing, etc, can put you at risk of cuts. Do be careful and wear protection such as proper gardening gloves when gardening.
  • Take extra care when brushing teeth or shaving to avoid cuts and bleeding gums. Consider using a soft toothbrush and an electric razor.
  • Try to avoid insect bites. Use a repellent when you are in contact with insects.

You cannot take an anticoagulant if you:

  • Are pregnant.
  • Have a stomach ulcer.
  • Have had a bleed into the brain (a haemorrhagic stroke).
  • Take certain medicines that may interfere with your anticoagulant. For example, certain medicines interact with warfarin and increase your risk of bleeding (for example, aspirin or ibuprofen).
  • Have major bleeding and this is not being treated.
  • Are going to surgery where you might be at risk of a major bleed.
  • Have very high blood pressure.
  • Have severely reduced kidney function - this is for dabigatran only.

For a full list of people who cannot take an anticoagulant, please see the leaflet that comes with your medicine.

No, they are only available from a pharmacist, with a doctor's prescription.

If you think you have had a side-effect to one of your medicines you can report this on the Yellow Card Scheme. You can do this online at the following web address:

The Yellow Card Scheme is used to make pharmacists, doctors and nurses aware of any new side-effects that medicines or any other healthcare products may have caused. If you wish to report a side-effect, you will need to provide basic information about:

  • The side-effect.
  • The name of the medicine which you think caused it.
  • The person who had the side-effect.
  • Your contact details as the reporter of the side-effect.

It is helpful if you have your medication - and/or the leaflet that came with it - with you while you fill out the report.

Further help & information

Anticoagulation Europe (UK)

PO Box 405, Bromley, Kent, BR2 9WP

Tel: 0208 289 6875

Further reading & references

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but makes no warranty as to its accuracy. Consult a doctor or other healthcare professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Jenny Whitehall
Current Version:
Dr Colin Tidy
Peer Reviewer:
Dr Hannah Gronow
Document ID:
13784 (v5)
Last Checked:
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