Warfarin is a type of anticoagulant medication taken in tablet form used to thin the blood. Warfarin is given to either prevent blood clots developing or to stop clots that have already developed from getting larger.

How does warfarin work?

Warfarin thins the blood by stopping blood clotting proteins from working. These proteins need a vitamin to work properly;this vitamin is called Vitamin K. We get Vitamin K from fruit and vegetables. Warfarin acts against Vitamin K and stops the blood clotting proteins from working properly and thins the blood.

Is blood thinning safe?

If the blood is too thin there is a risk of bleeding. That is why the level of blood thinning needs to be checked regularly.

How are blood thinning levels measured?

Blood thinning levels are measured with a blood test called the INR (international normalized ratio). Someone with “normal” blood thinning has an INR of 1.0. Most people on warfarin require blood thinning to a level of an INR 2.0 – 3.0. Sometimes patients require “thinner” blood levels with INR levels of 2.5 – 3.5.

How often do I need INR levels checked?

Once warfarin is started, you will need frequent blood tests – usually every second day. This will help find out how much warfarin you require. Once you have “stable” levels and your dose is known, blood tests are usually done once every few weeks.

How are blood tests performed?

A number of pathology companies can check the warfarin level (e.g Melbourne Pathology, Healthscope and Dorevitch). There are a number of pathology collection centres around Melbourne and INR blood tests can be done each of these collection centres.

What dose of warfarin should I take?

Different people require different doses of warfarin and this will depend on the level of blood thinning required. Some people need only a little amount of warfarin (e.g. 1mg per day) but other people need much more warfarin (greater than 10mg per day). It will take a week or two and a few blood tests to find out how much warfarin you will need.

Should I change my diet when on warfarin?

The amount of warfarin you require will be altered depending on how much vitamin K (fruit and vegetables) you have in your diet. It is not essential to change your diet but it is helpful to have a healthy diet that is generally stable in the amount of fruit and vegetables you eat. If you change your vitamin K intake significantly (e.g. change the amount of fruit and vegetables you eat) then the amount of warfarin you need may also change.


The information presented in this fact sheet is intended as a general guide only. Patients should seek further advice and information about Warfarin and or their individual condition from their treating haematologist or doctor.

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