Vitamin K

Can you take vitamin K if you take warfarin?

Updated 3 Nov 2026

Short answer

Only after talking to the doctor who manages the warfarin. Warfarin works by blocking vitamin K, so a sudden change in vitamin K from food or supplements can make it work more or less strongly.[1] People who take warfarin need to keep their vitamin K intake steady.[1] The authors of one study warned that 50 µg of MK-7 a day or more may change the effect of warfarin.[2]

How warfarin and vitamin K interact

  1. The liver needs vitamin K to make active clotting factors, such as prothrombin.[1]
  2. Warfarin, phenprocoumon and acenocoumarol block vitamin K. The liver then makes fewer active clotting factors, and blood clots more slowly.[1]
  3. A sudden rise or fall in vitamin K intake can make the medicine weaker or stronger.[1]
  4. The effect of warfarin is followed with a blood clotting test, the INR (international normalized ratio).[3]

Most multivitamins contain vitamin K, usually less than 75% of the daily value. Some other supplements with vitamin K contain very high amounts, such as 4,050 µg.[1]

MK-7 and warfarin

MK-7 stays in the blood much longer than vitamin K1 and builds up with daily intake. A study in healthy volunteers measured this. Its authors warned that 50 µg of MK-7 a day or more may change the effect of oral blood thinners in a way that matters.[2]

Can a small daily dose make the INR steadier?

In some people who take warfarin, the INR goes up and down for no clear reason. These people eat less vitamin K than people with a steady INR.[3]

  • UK trial. 70 people with an unsteady INR took 150 µg of vitamin K or a placebo every day for 6 months. The share of time with the INR in the target range rose by 28% with vitamin K and by 15% with placebo.[3]
  • Canadian trial. People on long-term warfarin took 150 µg of vitamin K or a placebo every day for 6 months. Time in the target range did not differ (65.1% against 66%), but very high or very low INR values were less frequent with vitamin K.[4]
  • Review. A 2016 review of 15 studies with 1,838 patients found that daily vitamin K may help people with an unsteady INR. In patients not chosen this way, the effect was small and did not matter in practice. The authors called for larger studies.[5]

In the Canadian trial, time in the target range also rose in the placebo group. The authors thought that closer monitoring of warfarin explained this.[4]

Other medicines that add to the effect

Orlistat, a weight-loss medicine, lowers the absorption of vitamin K. Taken with warfarin, it may make blood take much longer to clot.[1]

Who should ask first

Anyone who takes warfarin or a similar medicine should ask their doctor or pharmacist first. This applies to starting, stopping or changing any supplement with vitamin K, including a multivitamin.[1]

Sources (5)

  1. Vitamin K: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements. ods.od.nih.gov
  2. Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7. Schurgers LJ, Teunissen KJ, Hamulyák K, Knapen MH, Vik H, Vermeer C Blood, 2007. PMID 17158229
  3. Vitamin K supplementation can improve stability of anticoagulation for patients with unexplained variability in response to warfarin. Sconce E, Avery P, Wynne H, Kamali F Blood, 2007. PMID 17110451
  4. The effect of low-dose oral vitamin K supplementation on INR stability in patients receiving warfarin. A randomised trial. Boonyawat K, Wang L, Lazo-Langner A, Kovacs MJ, Yeo E, Schnurr T, et al. Thromb Haemost, 2016. PMID 27346552
  5. Effect of Vitamin K Intake on the Stability of Treatment with Vitamin K Antagonists: A Systematic Review of the Literature. Dentali F, Crowther M, Galli M, Pomero F, Garcia D, Clark N, et al. Semin Thromb Hemost, 2016. PMID 27232386

More about vitamin k

What is vitamin K?Which foods contain vitamin K?What is vitamin K used for?Who is at risk of vitamin K deficiency?Is vitamin K safe?Why do newborn babies get vitamin K?

For education; not medical advice. Talk to a doctor before you start, stop or change a supplement.