What is Warfarin?

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Warfarin is an anticoagulant drug that is also known under the brand names of Coumadin, Jantoven, Marevan, Lawarin, and Waran.

Initially marketed as a pesticide for rodents, warfarin was soon found to prevent the occurrence of both thrombi and emboli. For over fifty years now, warfarin has been approved for medical use and continues to be a commonly used anticoagulant drug throughout the world today.

How does warfarin work?


Warfarin works to decrease blood coagulation and, therefore, lower the risk of thrombosis and embolism, by inhibiting the enzyme that is responsible for the oxidation of vitamin K in the liver. Vitamin K is important for the normal process of coagulation, which is why warfarin affects the process of blood clotting and reduces the risk of thrombosis.

Benefits and risks

When making the therapeutic decision as to whether warfarin is justified, it is prudent to consider both the benefits of risks of taking this medication.

The primary benefit of warfarin is the reduction of clotting factors and the related decreased risk of thrombosis. The value of this benefit varies according to the likelihood that the patient might suffer from this condition and the severity if it does occur. People with a family history of deep venous thrombosis (DVT), for example, may benefit more from the use of warfarin, as well as those who have concurrent medical conditions, such as heart disease or a previous stroke.

This benefit must then be weighed up against the risks that warfarin may pose to the patient. The most important consideration is an increased risk of bleeding due to the way the medication works. As a result, patients taking warfarin may notice that they bruise more easily and cuts take longer to stop bleeding. If the individual is likely to suffer from bleeds, such as an elderly person who falls regularly, the risk of taking warfarin is greater.

Side effects

Some of the potential side effects of warfarin use may include:

  • Abdominal pain
  • Bloating
  • Chills
  • Fever
  • Gastrointestinal bleeding
  • Headache
  • Long nosebleeds
  • Nausea and vomiting
  • Taste changes

Drug interactions

Warfarin is well known for the vast number of medications with which it interacts, as it exerts its effect in the liver where most drugs are metabolized.

It is wise to double-check the medication history of any patient who may be advised to begin therapy with warfarin, including over-the-counter products. Some herbal medications, such as St. John’s Wort, can also change the concentration of vitamin K in the blood and have a dramatic effect on bleeding and coagulation.

Additionally, some food sources can interact with warfarin, such as grapefruit and cranberry juice. Foods high in vitamin K like spinach can also affect clotting factors and the effect of warfarin.

Patient advice

When a patient begins taking warfarin therapy, it is important that they are adequately advised on the possible effects of this drug.

Warfarin should be taken orally once a day at the same time, although taking it with or without food does not make a difference. It is important that the chosen brand of warfarin remains the same, as the effect of different brands may vary slightly. If a dose is missed, it should be taken as soon as the patient remembers, although two doses should never be taken together if not realized until the next day.

Frequent blood tests will be required, especially at the commencement of therapy, to determine the correct dose of warfarin. The international normalized ratio (INR) is used to quantify clotting factors and the efficacy of the dose.

Patients should be advised to continue eating a normal, healthy diet. Although some foods do affect the efficacy of warfarin, patients who eat a consistent diet each week can adjust their dosage. Unusually high quantities of green leafy vegetables or grapefruit may change how well warfarin works.


Further Reading

Last Updated: May 13, 2023

Yolanda Smith

Written by

Yolanda Smith

Yolanda graduated with a Bachelor of Pharmacy at the University of South Australia and has experience working in both Australia and Italy. She is passionate about how medicine, diet and lifestyle affect our health and enjoys helping people understand this. In her spare time she loves to explore the world and learn about new cultures and languages.


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