How anticoagulants work 

Anticoagulant medicines work by interrupting part of the process involved in the formation of blood clots. This means that blood clots are less likely to form where they are not needed, but can still form when they are.


Warfarin is a commonly prescribed anticoagulant medicine that is taken orally (swallowed) as a tablet. It interferes with your body's natural chemical processes by targeting a substance called vitamin K.

Vitamin K has an essential role to play in the production of prothrombin, a protein found in the blood. Prothrombin plays an important part in the process of the formation of clots.

If the production of vitamin K is slowed down, the production of prothrombin is also slowed. This means that it will take longer for blood clots to form.


Heparin occurs naturally in the body, but it can also be extracted and purified to be used as an anticoagulant medicine. Heparin inhibits the blood clotting process.

It can be given as an injection or through a drip into a vein. Different types of heparin medication include:

Monitoring your dose

How well your warfarin is working is measured using the international normalisation ratio (INR).

While you are taking warfarin, your INR will be regularly tested by your GP, pharmacist or by a nurse at your local hospital to make sure that the dose is correct.

  • If your INR is too high, blood clots will not form quickly enough and you may experience bruising or be at increased risk of bleeding. In this case, your dose may need to be reduced.
  • If your INR is too low, your medication is not working sufficiently, which means that clots could still form unnecessarily and block a blood vessel. In this case, your dose may need to be increased.

When you first start taking warfarin, your INR will be tested within the first two to four days of starting treatment. Depending on your reading, further tests will be carried out one to two times a week. If your INR stabilises within the correct range, testing may become less frequent.

International normalisation ratio (INR)

The INR is a way of measuring how fast your blood clots. As INR is an internationally recognised test, it can be used by healthcare professionals around the world.

During the test, a sample of your blood is taken and a chemical is added to it. The chemical starts a chain of chemical reactions that should make the blood in the sample clot (thicken).

During the clotting process, a protein in the blood (prothrombin) turns into an enzyme called thrombin. The time that it takes the prothrombin to turn into thrombin is called the prothrombin time ("pro-time", or PT). This is measured in seconds.

Your PT is compared with the PT of someone who is not taking warfarin. This gives your INR. Once the warfarin begins to work, your INR should start to increase. Normal INR ratings should be:

  • between 0.8 and 1.2 for people who are not taking warfarin
  • between 2.5 and 3.5 for people who are on warfarin, depending on the reason for taking it

New medicine service (NMS)

If you are prescribed a medicine to treat a long-term condition for the first time, you may be able to get extra help and advice about your medicine from your local pharmacist through a new free scheme called the New medicine service (NMS).

Media last reviewed: 16/09/2013

Next review due: 16/09/2015

Other anticoagulant medicines

There are several anticoagulant medicines. A full list can be found in our anticoagulant medicines information pages.

A combination of different anticoagulant medicines may also be used as part of your treatment.

Page last reviewed: 04/09/2013

Next review due: 04/09/2015