Drug & supplement interaction

Vitamin K + Warfarin

Warfarin brand name: Warfarin.

Major Interaction
Medically reviewed by · Updated October 2026
The bottom line Major

Vitamin K has a major interaction with Warfarin. Vitamin K can antagonize and reverse the therapeutic effects of warfarin.

Read the full interaction report
SeverityMajor
LikelihoodLikely
EvidenceC
What do Severity, Likelihood & Evidence mean?
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Interaction report

Vitamin K and Warfarin

Here is the interaction we found between these two, with the evidence behind it.

Major

WARFARIN (Coumadin)

Vitamin K can antagonize and reverse the therapeutic effects of warfarin.
Vitamin K antagonizes the effects of warfarin. Excessive vitamin K intake, either from supplements or from changes in the diet, can reduce the anticoagulant effect of warfarin.
Severity Major Likelihood Likely Evidence C

Well-controlled human studies have demonstrated the likely existence of this interaction.

Dr. Brian Staiger, PharmD, BCPS
Dr. Brian Staiger explains PharmD, Clinical Pharmacist

Here's the key thing to understand: warfarin works by lowering your body's supply of active vitamin K, which is what your liver needs to build clotting factors. So warfarin and vitamin K are basically tug-of-war partners. When you add extra vitamin K (from a supplement or a sudden jump in leafy greens like kale and spinach), you're handing the other side more rope. That can weaken warfarin's blood-thinning effect and push your INR down, raising your risk of an unwanted clot, the very thing warfarin is meant to prevent.

Warfarin is a narrow therapeutic index drug, so even small, steady changes matter. The goal isn't zero vitamin K, it's keeping your intake consistent. Please talk with your pharmacist or doctor before starting, stopping, or changing a vitamin K supplement so they can monitor your INR.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. McEvoy GK, ed. AHFS Drug Information. Bethesda, MD: American Society of Health-System Pharmacists, 1998.
  2. Crowther MA, Ageno W, Garcia D, et al. Oral vitamin K versus placebo to correct excessive anticoagulation in patients receiving warfarin: a randomized trial. Ann Intern Med. 2009;150(5):293-300. PubMed
  3. Dietary vitamin K guidance: an effective strategy for stable control of oral anticoagulation? Nutr Rev. 2010;68(3):178-81. PubMed
  4. Kim JS, Nafziger AN, Gaedigk A, et al. Effects of oral vitamin K on S- and R-warfarin pharmacokinetics and pharmacodynamics: enhanced safety of warfarin as a CYP2C9 probe. J Clin Pharmacol. 2001 Jul;41(7):715-22. PubMed
  5. Dezee, K. J., Shimeall, W. T., Douglas, K. M., Shumway, N. M., and O'malley, P. G. Treatment of excessive anticoagulation with phytonadione (vitamin K): a meta-analysis. Arch.Intern.Med. 2-27-2006;166(4):391-397. DOI
  6. Dentali, F., Ageno, W., and Crowther, M. Treatment of coumarin-associated coagulopathy: a systematic review and proposed treatment algorithms. J.Thromb.Haemost. 2006;4(9):1853-1863. PubMed

What These Ratings Mean

Every interaction above is graded three ways — here’s the key to each label.

Severity how clinically significant the interaction is
Major

Clinically significant. Avoid the combination, or use only under close professional supervision.

Moderate

Usually worth avoiding or managing — may need monitoring, a dose change, or timing apart.

Minor

Generally not serious. Be aware of it and mention it to your pharmacist.

Likelihood how strong the evidence is that the interaction actually happens in people
  • LikelyWell-controlled human studies have demonstrated the likely existence of this interaction.
  • ProbableInteraction has not been documented in well-controlled studies, however, the interaction has been demonstrated in some small human studies or in controlled animal studies in conjunction with multiple case reports.
  • PossibleInteraction has been documented in animal or in lab research, or the interaction has been documented in humans but is limited to case reports or conflicting clinical research exists.
  • UnlikelyInteraction has been demonstrated in animal or in lab research but has been shown not to occur in humans.
Evidence the quality of the studies behind the rating (graded A–D)
  • AHigh-quality meta-analysis (quantitative systematic review).
  • BCase-control study.
  • CConsensus.
  • DAnecdotal evidence.

These ratings are for information only. Never start, stop, or otherwise change your therapy before speaking with your provider.

Keep it in perspective

Vitamin K Also Interacts With Other Medications

Vitamin K has 1 major, known drug interaction in our database — the Warfarin one above is one of them. If you take anything else, check it against the full list below.

Major 1 No known interaction 2,829 (incl. Warfarin)
Taking anything else?

Check Another of Your Medications Against Vitamin K

Covers the 2,830 medications we’ve checked against Vitamin K. Pick one to open its full report.

From our Q&A

Questions About Vitamin K or Warfarin

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers Vitamin K or Warfarin, not necessarily both, but if one is close to what you were about to ask it will usually go further than a database entry can:

Not quite your question? Ask our pharmacists directly — it is free, there is no account required, and answers usually come back within 24 hours.

Everything else you take

See Every Supplement That Interacts With Warfarin

This page covers Vitamin K only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Warfarin page lists 361 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), Alfalfa (major), American Ginseng (major), Beer (major) and Cannabis (major).

By severity: 16 major · 301 moderate · 44 minor.

From our pharmacist’s overview on the Warfarin page

“People take warfarin to prevent dangerous clots, and many are the same people reaching for glucosamine for sore joints or a multivitamin containing vitamin K, which makes this a list that deserves genuine respect. Of the 361 supplements on file for warfarin, 16 carry a major rating, and the danger runs in both directions: some add to w…”

Reviewed by 2 · Jul 18, 2026

Open the Warfarin interaction page

One more smart check

Taking Vitamin K as Part of a Dietary Supplement Product? Check It for Interactions

We hold 5,292 real supplement products containing Vitamin K, each one broken down to its full ingredient list — and combination products often carry several ingredients besides Vitamin K. Your product may carry other ingredients that interact with Warfarin too, on top of Vitamin K — and those would not show on this page. Find your product and we’ll check every ingredient on its label:

How much people look this up

Where Vitamin K and Warfarin Sit Against Everything Else

Out of every supplement and medication people look up on HelloPharmacist, here is where these two land this week — and which way each has been moving.

Supplement Vitamin K
Top 5%

#26 of 1,452 supplements looked up here in the last 7 days

29 days ago today
Medication Warfarin
Top 10

#4 of 2,830 medications looked up here in the last 7 days

29 days ago today

Position among everything looked up on this site, not a measure of how commonly either one is used or how important it is. A supplement can climb this list because of a news story. It counts every request to the page, automated ones included, and nothing here identifies anyone.

Supplement

About Vitamin K

What is Vitamin K?

  • Supporting normal blood clotting
  • Bone health
  • Heart and vascular health
  • Correcting vitamin K deficiency
  • Newborn bleeding prevention (medical injection)

Vitamin K is an essential nutrient your body needs for normal blood clotting and to support healthy bones. Most people get enough from food, but supplements are sometimes used for deficiency or bone and heart health, where evidence is mixed. If you take a blood thinner, talk to your pharmacist or doctor before changing your vitamin K intake.

Read the full Vitamin K monograph & all interactions
Medication

About Warfarin

What Is Warfarin Used For?

Brand name: Warfarin

Warfarin helps prevent and treat harmful blood clots. The label lists these uses for both Warfarin Sodium tablets and Jantoven tablets:

  • Preventing and treating venous thrombosis (clots in the veins) and pulmonary embolism (clots in the lungs).
  • Preventing and treating clot-related complications of atrial fibrillation and/or heart valve replacement.
  • Lowering the risk of death, another heart attack, and clot-related events such as stroke after a heart attack.

Warfarin does not dissolve a clot that has already formed. It helps stop the clot from growing and prevents further clot-related complications.

See full Warfarin drug information
Keep checking

Other Drugs & Vitamin K

Browse every other medication we’ve checked against Vitamin K — both the ones with a known interaction and the ones without. Select any drug for the full detail.

1Major
0Moderate
0Minor
2,828No known interaction
1 Major 2,828 No interaction
Major

1 interaction

No interaction 2,828 drugs checked — show the list

We also checked Vitamin K against these medications and found no known interaction in our sources. Open any page to confirm.

Showing the 60 most looked-up of 2,828 medications checked with no known interaction — see the full Vitamin K list.

See the full Vitamin K interactions page
See every supplement checked against Warfarin, ranked by severity
Sources

Sources & How We Checked

Interaction details are evidence-graded and sourced from the Natural Medicines database. The medication overview comes from MedlinePlus.

Content is written and reviewed by licensed HelloPharmacist pharmacists. See our data sources and editorial standards for how this information is built and checked.

Parts of this content are provided by the Therapeutic Research Center, LLC.

DISCLAIMER: Currently this does not check for drug-drug interactions. This is not an all-inclusive comprehensive list of potential interactions and is for informational purposes only. Not all interactions are known or well-reported in the scientific literature, and new interactions are continually being reported. Input is needed from a qualified healthcare provider including a pharmacist before starting any therapy. Application of clinical judgment is necessary.

© 2021 Therapeutic Research Center, LLC

Keep exploring
Vitamin K monograph › Uses, safety, and every drug interaction in one place Warfarin supplement interactions › Every supplement checked against Warfarin, ranked by severity Warfarin drug information › The full plain-English guide to this medication Supplement interaction checker › Check any supplement or product against your medications Browse medications A–Z › Every drug we've checked against herbs & supplements Ask a pharmacist › Free, personal answer from our licensed team
Pharmacist Counseling Corner

Taking Vitamin K and Warfarin Together: Common Questions

Can you take Vitamin K with Warfarin?
This is rated a major interaction, so the combination is best avoided unless your doctor or pharmacist has specifically told you otherwise. Read the full details on this page and confirm with your pharmacist before combining them.
How serious is the Vitamin K and Warfarin interaction?
We rate it Major. This is a clinically significant combination that is generally best avoided, or used only under the direct supervision of your physician or pharmacist.

Written and reviewed by the HelloPharmacist editorial staff. Our editorial policy

Still Have a Question About Warfarin and Vitamin K?

Our pharmacists answer your medication & supplement questions — free.

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This information is for education only and is not a substitute for professional medical advice. Always check with your pharmacist or doctor before starting, stopping, or combining supplements and medications.