Does vitamin K interact with warfarin? (3+ interaction)

In this article, we will discuss the interaction between vitamin K and warfarin, along with the dietary products to avoid while consuming warfarin. Additionally, we will also discuss the precautionary measures to take while on warfarin therapy. 

Does vitamin K interact with warfarin? 

Yes, excess intake of vitamin K or a diet containing vitamin K can increase the risk of bleeding. A sudden increase in the amount of vitamin K can reduce the anticoagulant effect of warfarin. (1)

Warfarin is an anticoagulant which is used to prevent blood clots. Warfarin acts by inhibiting the vitamin K epoxide reductase complex 1 (VKORC1) enzyme and prevents vitamin K activation. Vitamin K is an important factor in blood coagulation as it activates coagulating factors and proteins C and S, which are essential for blood clotting. (2)

Warfarin is used as a prophylaxis and in the treatment of venous thrombosis, pulmonary embolism, atrial fibrillation, cardiac valve replacement, recurrent myocardial infarction, etc. (2)

Vitamin K is a coenzyme which is essential in blood clotting, bone metabolism and other physiological functions. Anticoagulants such as warfarin decrease the activity of vitamin K and reduce coagulation. (3)

What does the research suggest? 

One of the case studies by G D Qureshi et al. reported warfarin resistance in a 31-year-old woman. The woman was on 35mg warfarin treatment and a weight-reducing diet. During the therapy, she developed resistance to warfarin. Her daily diet contained 1,277mcg of vitamin K. (4)

When her daily intake of vitamin K was decreased to 360mcg, which is less than her usual intake, the warfarin resistance was reduced. Hence, the study concluded that the resistance to warfarin was because of a higher intake of dietary vitamin K. (4)

Another case study by F, B Walker reported a 35-year-old woman who suffered from myocardial infarction (MI) during her warfarin therapy. She was prescribed a vegetable-rich diet of lettuce, broccoli and turnip greens for weight loss, following which she developed MI after five weeks of diet intake. The study concluded the excess amount of vitamin K caused a life-threatening condition in the patient. (5)

Anticoagulation instability was seen due to a change in the intake of vitamin K in patients with long-term oral anticoagulants. This can be avoided with careful intake of a vitamin K-rich diet and multivitamins. (6)

How does vitamin K affect warfarin activity? 

Vitamin K is a main cofactor that plays an important role in blood clotting. Warfarin, on the other hand, is an anticoagulant that prevents the clotting of blood. Anticoagulant therapies are used to prevent thrombosis, embolism and MI in patients who are at a greater risk. (2,3)

Whenever an anticoagulant therapy is initiated, the Internation Normalised Ratio (INR) should be maintained in the range of 2-3. However, with excess vitamin K intake, the action of warfarin decreases, and changes in the INR are seen. Maintaining an INR of in between the range of 2-3 is important to prevent the risk of life-threatening conditions such as MI, stroke or excessive bleeding (7)

Research studies have shown that increased baseline plasma concentrations of vitamin K led to a slow rise in the INR value, and consuming more vitamin K-rich diet or supplements (>250µg per day) can decrease the sensitivity to warfarin and can lead to warfarin resistance. (1)

What dietary intake should be avoided while on warfarin therapy? 

There are certain diet restrictions which need to be followed while on warfarin therapy to avoid the risk of complications. Vitamin K is necessary to carry out bodily functions, but an excess amount of vitamin K during warfarin therapy can increase the risk of bleeding. The following are sources of vitamin K, which should be eaten in smaller amounts.  

Diet Vitamin K content
Leafy vegetables: dark green and deep yellow 50-800 µg of vitamin K per 100 g
Milk, dairy products, eggs, fruits, and cereals 5-50µg of vitamin K per 100 g
Animal liver porcine 25µg per 100mg, bovine 92µg per 100g 
Natto, 3 ounces (as MK-7) 850 µg
Spinach, raw, 1cup 145 µg 
Kale, raw, 1 cup 113 µg
Broccoli, chopped, boiled, 1/2 cup 110 µg
Soybeans, roasted, 1/2 cup 43 µg
Carrot juice, 3/4 cup  28 µg

It is important to monitor the amount of dietary intake of vitamin K while on warfarin therapy to avoid complications. 

How to use warfarin safely and effectively? 

Using warfarin safely and effectively involves following proper instructions, such as

  • Always take the dose of warfarin as prescribed by your physician.
  • Do not change or stop taking the dose of warfarin without consulting your physician. 
  • Warfarin requires frequent monitoring, especially INR. Attend all scheduled appointments to ensure proper assessment of your treatment. 
  • Maintain a consistent intake of vitamin K. Do not consume a large amount of vegetables such as spinach, green vegetables, animal liver, etc. Vitamin K-rich food can interact with warfarin and reduce its action. Have a discussion with your physician and plan a proper diet. 


  • OTC drugs such as cold medication, supplements and herbal products can interact with warfarin and can increase the risk of bleeding. 
  • Avoid taking multivitamin tablets as it can increase the plasma concentration of vitamin K and decrease the action of warfarin. 
  • The use of warfarin comes with a risk of increased bleeding. Look out for any bruises and avoid activities which can put you at risk of bleeding. 
  • Limit the use of alcohol and tobacco as it can interfere with the use of warfarin and its metabolism. 
  • Consult your physician if you notice any unwanted changes while on warfarin treatment. 

According to my knowledge and experience as a pharmacist, excess intake of vitamin K can interfere with the action of warfarin and increase the risk of bleeding, thrombosis, embolism and MI. It is important to maintain a proper and balanced diet while on warfarin. Avoid taking any vitamin supplements without your physician’s approval. 


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Lurie Y, Loebstein R, Kurnik D, Almog S, Halkin H. Warfarin and vitamin K intake in the era of pharmacogenetics. British journal of clinical pharmacology. 2010 Aug;70(2):164-70. Available from: 


Patel S, Singh R, Preuss CV, et al. Warfarin. [Updated 2023 Mar 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: 


National Institutes of Health (NIH) Office of Dietary Supplements (ODS). Vitamin K
Fact Sheet for Health Professionals [Internet]. Bethesda, MD. 1995 [updated: March 29, 2021, cited: December 30 2023]. Available from: 


Qureshi GD, Reinders TP, Swint JJ, Slate MB. Acquired warfarin resistance and weight-reducing diet. Archives of internal medicine. 1981 Mar 1;141(4):507-10. Available from: 


Walker FB. Myocardial infarction after diet-induced warfarin resistance. Archives of internal medicine. 1984 Oct 1;144(10):2089-90. Available form: 


Chow WH, Chow TC, Tse TM, Tai YT, Lee WT. Anticoagulation instability with life-threatening complication after dietary modification. Postgraduate medical journal. 1990 Oct;66(780):855-7. Available from: 


Shikdar S, Vashisht R, Bhattacharya PT. International Normalized Ratio (INR) [Updated 2023 May 1]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: