Interaction fact sheet
Vitamin K and VKA anticoagulants
Monitor: consistency of intake matters more than quantity.
Vitamin K counteracts the effect of vitamin K antagonist anticoagulants, such as warfarin or fluindione. Taking a supplement without telling your healthcare team can lower INR. Cutting out green vegetables, however, is not useful: keeping intake consistent is better.
In brief
- Mechanism
- Vitamin K antagonists (VKAs) block the recycling of vitamin K, which is needed for blood clotting. A sudden increase in vitamin K intake reduces their effect.
- Magnitude
- Effect mainly above around 150 µg of vitamin K per day, and with large changes in intake.
- Who is affected
- People taking warfarin, fluindione or acenocoumarol. Direct oral anticoagulants are not affected by this mechanism.
- What to do
- Keep intake consistent, do not start a vitamin K or K2 supplement without advice, and check INR if anything changes.
Should you cut out green vegetables?
No. A 2016 systematic review concluded that the evidence does not justify restricting dietary vitamin K while taking vitamin K antagonists. An effect on clotting appears mainly with high intakes, above around 150 µg per day, and with sudden changes. What destabilizes INR is variation, not your usual intake.
What about supplements?
A vitamin K supplement, or vitamin K2 often combined with vitamin D, introduces a new, regular dose that can lower INR. Paradoxically, in a trial involving 70 patients with unstable INR, 150 µg of vitamin K daily, under medical supervision, improved treatment stability. This is a medically supervised strategy, not self-medication.
A review of 120 medicines and foods that interact with warfarin notes that many herbs and supplements can also alter INR.
In practice
What you can do
- Keep your eating habits consistent, including green vegetables.
- Don't start or stop a vitamin K or K2 supplement without talking to your doctor.
- Mention any new supplement: many herbs change the effect of VKAs.
When to seek advice
- Nosebleeds or bleeding gums, unusual bruising, dark urine or stools: seek medical advice promptly.
- Major dietary change: dieting, fasting, travel.
- Wanting to take a vitamin D supplement combined with K2.
Main sources
- 01Violi F et al. Interaction between dietary vitamin K intake and anticoagulation by vitamin K antagonists: is it really true? A systematic review. Medicine, 2016. PubMed
- 02Sconce E et al. Vitamin K supplementation can improve stability of anticoagulation for patients with unexplained variability in response to warfarin. Blood, 2007. PubMed
- 03Holbrook AM et al. Systematic overview of warfarin and its drug and food interactions. Archives of Internal Medicine, 2005. PubMed
Frequently Asked Questions (FAQ)
Can you eat spinach while taking anticoagulants?
Yes, even if you're taking a VKA. What matters is keeping your intake consistent from week to week, rather than alternating between large portions and none at all.
Is vitamin K2 dangerous with warfarin?
It can reduce warfarin's effect. Do not take it without medical advice and INR monitoring.
Are newer anticoagulants affected?
Direct oral anticoagulants such as apixaban or rivaroxaban do not work through vitamin K. They have other interactions: check with your pharmacist.