Warfarin belongs to a class of drugs called anticoagulants. Anticoagulants thin blood and make it less susceptible to clotting. Warfarin comes in the form of tablets. It takes several days for warfarin to become fully effective and the first several days have to be covered by a bridging therapy in the form of injections of low molecular weight heparin (LMWH), e.g. tinzaparin, dalteparin or enoxaparin. There are now some newer oral anticoagulants (NOAC) such as dabigatran, apixaban and rivaroxaban available.

Anticoagulant drugs are prescribed to patients who already developed blood clots (thrombi) or who at risk of developing them. Some of the conditions where you may need the prescription of warfarin include:

  • Deep vein thrombosis (DVT) - blood clots in the deep veins of legs
  • Pulmonary embolism (PE) - shortness of breath due to blood clots in the lungs coming from other sites
  • Atrial fibrillation (AF) - fast and irregular heart beat
  • Prosthetic heart valves – artificial mechanical heart valves

The duration of treatment depends upon the condition for which it is prescribed. For deep vein thrombosis and pulmonary embolism it will be usually several months, atrial fibrillation and prosthetic heart valves require anticoagulation for life.

You will be given a yellow leaflet containing all the information about anticoagulants. Warfarin is taken once a day, especially in the evening. It should be taken at the same time of the day to avoid side effects or the complications of the disease for which it is prescribed.

Mode of action and monitoring

Warfarin blocks production of vitamin K which is necessary for the synthesis of proteins that are necessary for normal blood clotting (coagulation). When your GP starts you on warfarin, he/she would advise you to run a test once or twice a week to check how long it will take your blood to clot. This test is called international normalisation ratio (INR). The frequency of the test will depend on the fluctuation of your INR.

When to avoid warfarin?

Warfarin should be avoided if you have any of the following condition:

  • Active bleeding or high risk of bleeding
  • Ulcer in your stomach or anywhere in the digestive tract
Warfarin is also contraindicated in pregnancy.


Warfarin has interactions with many commonly used medicines and it is important to check your INR more often when your other medication changes. Some food also interfere with warfarin, mainly leaf vegetables with high content of vitamin K. Avoid excessive alcohol while you are on warfarin as it may trigger bleeding.

Adverse effects

As warfarin is a blood thinner it may lead to bruising and heavier bleeding in case of injuries. Patients on warfarin should protect themselves against injuries and cuts by:

  • being careful while shaving or brushing your teeth
  • protecting themselves with appropriate clothing and insect repellents against insect bites
  • taking care while sewing, gardening or participating in contact sports and games