Authored by , Reviewed by Dr Jacqueline Payne on | Certified by The Information Standard

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 (DVT), can lead to serious problems. For example, it can lead to a clot that travels from a leg vein to the lungs (a pulmonary embolism). Anticoagulants are used to prevent blood clots as well - the most common condition for this is atrial fibrillation (AF).

A number of anticoagulants are available, including warfarin, acenocoumarol, phenindione, dabigatran, apixaban, edoxaban 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, edoxaban and rivaroxaban are newer types of anticoagulant which do not require regular blood tests and are being increasingly used. They are referred to as 'new oral anticoagulants', or NOAC for short.

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 the separate leaflet called Aspirin and Other Antiplatelet Medicines for more details.

The newer anticoagulant medicines (dabigatran, apixaban, edoxaban and rivaroxaban) are now being used more than the older anticoagulant, warfarin. Dabigatran, apixaban, edoxaban and rivaroxaban have a number of advantages over warfarin, although they still have similar kinds of side-effects. Saying this, there are still some scenarios in which one of the older anticoagulants (most likely, warfarin) are used.

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 which has not been injured or cut.

Anticoagulants 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, edoxaban 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 four medicines work quickly - within two to four hours.

Anticoagulants are prescribed if you already have a blood clot, the most common cause being a deep vein thrombosis (DVT) and/or a clot on the lung, called a pulmonary embolus (PE). in these cases, they prevent the clot from becoming bigger. The other reason they are used is 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

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. Warfarin was the main anticoagulant for a number of years but is fast being overtaken by the newer oral anticoagulants. Warfarin may still be the anticoagulant of choice in certain clinical situations.

Dabigatran, edoxaban, rivaroxaban and apixaban

These are all now for people with a certain type of AF. In the UK, the National Institute for Health and Care Excellence (NICE) states that you should be able to have these as an option if you have a type of AF - known as non-valvular - and other risk factors for a stroke, determined by scoring systems (a common one is the CHADS VASc score).

Some of these medications are used in patients who develop blood clots, such as deep vein thrombosis (DVT) and pulmonary embolism, and also to prevent blood clots in patients having certain types of surgery.

Some differences between warfarin and newer anticoagulants

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, edoxaban 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, edoxaban 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. Warfarin can interact with a number of medications. For example, certain antibiotics can result in an enhanced warfarin effect which can then lead to bleeding. 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. Before health professionals prescribe you any anticoagulant, make sure you discuss with them all the medications you are taking.

Warfarin, acenocoumarol and phenindione can also interact with foods. Some foods contain vitamin K and if lots of this is eaten then the effect of warfarin, acenocoumarol and phenindione maybe reduced. Foods containing vitamin K are generally 'green', and include kale, spinach and Brussels sprouts. On the other hand, alcohol, grapefruit and cranberry juice can enhance the effect of these medications. Any prolonged periods of illness leading to a reduced oral intake may also lead to problems with these medications and may necessitate an earlier blood test to see how thin the blood is.

Some of the newer anticoagulants (dabigatran, apixaban, rivaroxaban and edoxaban) have less interaction with foods, alcohol and other medicines.

Unlike warfarin, the dose of the newer anticoagulants usually does not change during treatment. There is usually no need to alter the dose, although lower doses may need to be used if you have chronic kidney disease.

The advantages of the older-type anticoagulants is that:

  • They have been used for decades.
  • They are known to work well.
  • Most of the possible side-effects are known.

Anticoagulants and bleeding

The safety of newer oral anticoagulants has been compared to warfarin in some big research studies. One study was in patients with DVT who were treated with either a NOAC or warfarin. The risk of major bleeding was similar in both groups. In other studies comparing patients at risk of AF-related stroke, the overall risk of bleeding among patients taking NOACs was either similar to or lower than that for patients on warfarin.

In addition, the NOACs do not increase the risk of bleeding into the brain (intracranial haemorrhage) by nearly as much as warfarin. 

If a bleed does occur:

  • In an overdose/excess treatment with warfarin, vitamin K acts as an antidote, although it takes some time to work.
  • Apixaban, rivaroxaban and edoxaban currently have no treatment to stop the effect of these medicines if you do have severe bleeding.
  • Dabigatran can be reversed quickly with idarucizumab (Praxbind®).
  • The newer agents do not stay in your body for as long as warfarin does. This means that any problem with bleeding disappears more quickly - usually in 12-24 hours.

What does taking warfarin involve?

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.

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 AF 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.
  • 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.

How to use the Yellow Card Scheme

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 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 reading and references

when taking this tablet did anyone get increased chest pain and more breathing issues than any other blood thinning medication?

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