Warfarin is the main oral anticoagulant used in the UK (oral means 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) such as:

Warfarin may also be prescribed for people:

  • with a condition that increases the risk of a blood clot developing, such as atrial fibrillation
  • with a prosthetic (replacement or mechanical) heart valve
  • who have had a heart attack

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.

Taking warfarin

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.

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. You may be monitored less frequently, depending on your readings.

Although there are now three new anticoagulants that don’t require regular monitoring – rivaroxaban, apixaban and dabigatran – most patients who need an anticoagulant will be prescribed warfarin.

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.

Some foods and drinks can interfere with the effect of warfarin if eaten in large amounts, including food high in vitamin K and cranberry juice.

It is also dangerous to binge drink or get drunk while 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 experience any unusual and persistent symptoms.

It is also important to avoid injury while 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 or stings

Read more information about the side effects of warfarin.

© Crown Copyright 2009

This site uses cookies. By continuing to browse this site you are agreeing to our use of cookies. Find out more here.