Anticoagulants, warfarin


Warfarin is the main oral anticoagulant used in the UK (oral means that it is taken by mouth). An anticoagulant is a medicine that stops blood from clotting.

Anticoagulant medicines are most commonly prescribed for people who have had a condition caused by a blood clot (thrombosis) or are at risk of developing one. These conditions include:

Warfarin may also be prescribed for people with a replacement or mechanical heart valve.

How long you will take warfarin depends on the condition for which it has been prescribed. If you are not sure, ask your GP. You will also be given a yellow booklet on anticoagulants, which explains your treatment.

Warfarin is taken once a day, usually in the evening. It is important to take your dose at the same time each day. Read information about what to do if you miss a dose or take an extra dose of warfarin.

How it works

The blood needs vitamin K to be able to clot. Warfarin slows the production of vitamin K in the body, which increases the time it takes for your blood to clot. This is sometimes described as 'thinning the blood, although it doesn’t actually thin your blood'.

Warfarin helps your blood to flow freely around your body and stops any clots forming in the heart or in the blood vessels.

Things to consider

While taking warfarin, your dose will be monitored once or twice a week using the international normalisation ratio (INR) which measures how long it takes your blood to clot. This may become less frequent depending on your readings.

It is important to avoid taking warfarin if you are pregnant or have certain health conditions including:

Read more about things to consider when taking warfarin and read the answers to some common questions about taking warfarin.


Warfarin can interact with many other medicines. The patient information leaflet that comes with your medicine should tell you if it’s safe to take with warfarin, but ask your GP or pharmacist if you’re unsure. It is also dangerous to binge drink or get drunk whilst taking warfarin as this increases the risk of excess bleeding.

Read more information about how warfarin interacts with alcohol and other medication.

Side effects

Warfarin can cause several side effects and you should see your GP if you are experiencing any unusual and persistent symptoms.

It is also important to avoid injury whilst taking any anticoagulant, as they make you more prone to bleeding if you are injured. Try to avoid minor injuries and cuts and grazes by:

  • taking care when brushing your teeth and shaving
  • using protection when gardening, sewing, or playing contact sports
  • using insect repellent to avoid insect bites and stings

Read more information about the side effects of warfarin.

Last updated: 31 August 2015

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