Warfarin belongs to a class of drugs called anticoagulants. Anticoagulants thin the 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. The first few 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. Warfarin has been replaced in most indications by direct oral anticoagulants (DOAC), such as apixaban, edoxaban, rivaroxaban, and dabigatran, which don't require monitoring with regular blood samples.

Anticoagulant drugs are prescribed to patients who already developed blood clots (thrombi) or who at risk of developing them. The indications for 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
  • Atrial fibrillation (AF) - fast and irregular heartbeat
  • 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 usually be several months, atrial fibrillation and prosthetic heart valves require anticoagulation for life. Warfarin is taken once a day, usually in the evening. It should be taken at the same time every day to avoid fluctuations in its blood-thinning efficacy.

Mode of action and monitoring

Warfarin blocks the production of vitamin K, which is necessary to synthesise proteins involved in blood clotting (coagulation). The dose of warfarin is different in different people and it can also change over time. Patients on warfarin, therefore, need regular blood samples to determine the efficacy of blood thinning. This test is called INR (international normalised ratio), and the target range for most indications, including atrial fibrillation, is 2.0-3.0. The frequency of the monitoring will depend on the fluctuation of INR. If INR is frequently outside the target range, one of the NOACs should be considered instead of warfarin.

Warfarin should be avoided in patients with active bleeding or high risk of bleeding, ulcerations in the stomach or elsewhere in the digestive tract, and pregnancy.


Warfarin has interactions with many commonly used medicines, and it is essential to check INR more often when other prescribed medications change. Some food also interferes with warfarin, mainly leaf vegetables with high content of vitamin K. Alcohol excess whilst on warfarin increases the risk of bleeding.

Side effects

As warfarin is a blood thinner, it may lead to bruising and bleeding in case of injuries. Patients on warfarin should protect themselves against injuries and cuts by being careful while shaving or brushing teeth, protecting themselves with appropriate clothing and insect repellents against insect bites, and taking care while sewing, gardening, or participating in contact sports and games. The risk of serious bleeding in a correctly prescribed and monitored anticoagulation is low and is more than offset by the reduced risk of thrombotic events.

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