Boots WebMD Partners in Health
Return To Boots

Stroke health centre

Anticoagulant medicines - How anticoagulants work

NHS Choices Medical Reference

NHS Choices Logo

Anticoagulant medicines work by interrupting part of the process that is 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. Warfarin 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, which is 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, your pharmacist or by a nurse at your local hospital to make sure that your dose is correct.

  • If your INR is too high, blood clots will not form quickly enough and you may experience bruising or you may 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 your warfarin begin to work, your INR should start to increase. Normal INR ratings should be:

  • between 0.8-1.2 for people who are not taking warfarin
  • between 2.5-3.5 for people who are on warfarin, depending on the reason for taking it
Medical Review: November 23, 2011
Next Article:

Popular slideshows & tools on BootsWebMD

woman looking at pregnancy test
Early pregnancy symptoms
donut on plate
The truth about sugar addiction
smiling african american woman
Best kept secrets for beautiful hair
couple watching sunset
How much do you know?
nappy being changed
How to change your baby's nappy
woman using moisturizer
Causes and home solutions
assorted spices
Pump up the flavour with spices
bag of crisps
Food cravings that wreck your diet
woman with cucumbers on eyes
How to banish dark circles and bags
probiotic shakes
Help digestion
polka dot dress on hangar
Lose weight without dieting