What are anticoagulants?

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 an artery or a vein (blood vessels). 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 deep vein thrombosis (a clot in the leg vein), can lead to serious problems such as a pulmonary embolism (a clot that travels from the leg vein to the lungs).

A number of anticoagulants are available including: warfarin, acenocoumarol, phenindione, dabigatran, 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 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.

At present (early 2012) most people who need an anticoagulant are prescribed warfarin. However, there is a strong possibility that this may change. This is because national guidelines now recommend that dabigatran and rivaroxaban (two newer anticoagulants) are options in certain circumstances. Dabigatran and rivaroxaban have a number of advantages over warfarin, although they still have similar kinds of side-effects. Other newer anticoagulants may also become available over the next year or so.

How and why does blood clot?

Within seconds of cutting a blood vessel, the damaged tissue causes platelets (tiny cells in the blood) 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.

How do anticoagulants work?

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 and rivaroxaban both prevent a blood chemical called thrombin from working, which in turn prevents fibrin from being made from fibrinogen. Dabigatran binds to thrombin and rivaroxaban stops thrombin from being made. Both medicines work quickly – within two to four hours.

When are anticoagulants used?

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:

  • Atrial fibrillation (fast irregular heartbeat). Having atrial fibrillation is one of the most common reasons for taking an anticoagulant.
  • A mechanical heart valve.
  • Endocarditis (infection of the inside of the heart).
  • Mitral stenosis (one of the valves in the heart does not fully open).
  • Certain blood disorders that affect how your blood clots (inherited thrombophilia, antiphospholipid syndrome).
  • Had surgery to replace a hip or knee.

Which anticoagulant is normally used?

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 you 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 Clinical 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 nonvalvular and at least one of the following applies:

  • You have had a stroke, transient ischaemic attack (mini stroke) 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.


Like dabigatran, until recently, rivaroxaban 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 as an option if you have a type of atrial fibrillation known as nonvalvular and at least one of the following applies:

  • You have congestive heart failure (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 (mini stroke) at some point in the past.

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 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 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.

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.

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. 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.

What is the usual length of treatment?

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 (deep vein thrombosis). 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.

What are the possible side-effects?

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.

What if I bleed whilst taking an anticoagulant?

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 faeces (stools). 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 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.

What else should I be aware of when taking an anticoagulant?

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.

Who cannot take an anticoagulant?

You cannot take an anticoagulant if you:

  • Are pregnant.
  • Have a stomach ulcer.
  • Have had a haemorrhagic stroke (where there has been a bleed into the brain).
  • 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.

Can I buy anticoagulants?

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

Further help and information

Anticoagulation Europe

PO Box 405, Bromley, Kent, BR2 9WP
Tel: 020 8289 6875 Web: www.anticoagulationeurope.org
A charity providing information and advice to people on oral anticoagulation treatment.

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 http://yellowcard.mhra.gov.uk

The Yellow Card Scheme is used to make pharmacists, doctors and nurses aware of any new side-effects that your medicines 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.
  • Information about 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.

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