Herb & supplement monograph

Vitamin K Drug Interactions, Uses, Effectiveness, Safety & More

Medically reviewed by Dr. Brian Staiger, PharmD, BCPS ·Updated July 2026

What is this page for?

First and foremost: how Vitamin K interacts with medications. The heart of this page is the interaction list — every drug Vitamin K is known to interact with, and how serious each one is.

But these pages have grown well beyond that into a full monograph — what Vitamin K is, what people use it for and how strong the evidence is, its safety and side effects, and answers to the questions we’re asked most — written and reviewed by the clinical staff at HelloPharmacist. It’s educational information from our licensed clinical databases, not medical advice, and we don’t sell or endorse products. Our editorial policy

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Reviewed by licensed pharmacists Sourced from Natural Medicines

On this page

Vitamin K Drug Interactions: The Bottom Line

From the HelloPharmacist Editorial Team · Updated July 2026

Vitamin K drug interactions involve only a small handful of medications, but the risk is serious and well proven for those few, so most people face little concern unless they take a blood thinner. Every listed interaction is rated major and backed by strong human evidence, and the concern centers on blood-thinning medication.

Interactions deserving the most attention

The interaction that matters most is with warfarin (Coumadin). Vitamin K works directly against warfarin's blood-thinning effect, and extra vitamin K from supplements, or even a shift in diet, can make warfarin less effective. This is well documented in human studies, and because warfarin is a narrow-margin medicine used to prevent dangerous clots, even a modest drop in its effect can have real consequences.

What the rest of the list means

Unlike many supplements, Vitamin K does not come with a long list of weak or theoretical interactions. Its risk is concentrated in one clear place: medicines that thin the blood by blocking vitamin K itself. For people who do not take this kind of medicine, the data shows no other known interactions.

Check your medications against Vitamin K

Based on HelloPharmacist’s Vitamin K interaction data and reviewed under our editorial standards.

Interaction report

Drugs that interact with Vitamin K

2 medications have a known interaction with Vitamin K, graded by severity. Select any drug for the full evidence-based detail.

2 major interactions. These combinations can be clinically significant — best avoided, or used only with close professional supervision. Always check with your pharmacist before combining them with Vitamin K.

Don’t want to scroll the list? Just ask.

Tell us the medications you take and we’ll check each one against Vitamin K using our pharmacist-reviewed interaction data.

AI summaries are generated from our Vitamin K interaction data for education only — always confirm with your pharmacist. How we use AI

2,830 drugs

Read The List Like a Pharmacist

What Severity, Likelihood & Evidence Mean

Severity — How Serious It Can Be

Major. Clinically significant; generally best avoided, or used only under direct professional supervision.
Moderate. May need monitoring, a dose adjustment, or separating the times you take each one.
Minor. Generally not clinically significant, but still worth noting and mentioning to your pharmacist.
No known interaction. Checked against our sources with nothing documented — not the same as proven safety.

Likelihood — How Well It’s Documented

Likely. Well-controlled human studies have demonstrated the likely existence of this interaction
Probable. Interaction 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.
Possible. Interaction 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
Unlikely. Interaction has been demonstrated in animal or in lab research but has been shown not to occur in humans.

Where This Data Comes From

Interaction records are evidence-graded and sourced from the Natural Medicines database (TRC Healthcare), the same reference used by pharmacists and hospitals.
Each drug listed above links to the full report for that exact Vitamin K combination — clinical detail, likelihood, evidence level, and citations.
Content is reviewed by licensed HelloPharmacist pharmacists — see our data sources and editorial standards.

The big picture

The kinds of drugs Vitamin K affects

Every type of medication (drug category) Vitamin K is known to interact with. Open any category for the detail — or search your exact drug in the checker above.

All 1 drug category Vitamin K interacts with

Warfarin (coumadin)

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.

Likelihood Likely Evidence C

Monograph

Vitamin K: Uses, Safety & Side Effects

Medically reviewed by Dr. Brian Staiger, PharmD, BCPS · Reviewed July 2026 ·

The bottom line

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.

Part used: Fat-soluble vitamin (K1 from plants, K2 from animal sources and bacteria)
Common forms: Capsules, softgels, tablets, drops, injections (medical use)

People commonly use it for

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

Popular and traditional uses — not proof it works. See “Uses & effectiveness” below for the evidence.

Safety at a glance

OverallLikely safe

Generally safe in amounts found in food and most supplements for healthy people.

PregnancyLikely Safe

When used orally in amounts that do not exceed the daily adequate intake level (AI). A tolerable upper intake level for vitamin K in pregnancy and lac...

Read the full pregnancy detail

BreastfeedingLikely Safe

When used orally in amounts that do not exceed the daily adequate intake level (AI). A tolerable upper intake level for vitamin K in pregnancy and lac...

Read the full breastfeeding detail

Pregnancy & breastfeeding ratings are from Natural Medicines (Therapeutic Research Center). Safety guidance is general; always confirm with your pharmacist or doctor for your situation.

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Overview

Vitamin K is a fat-soluble vitamin that your body needs to make proteins involved in blood clotting and bone health. There are two main natural forms. Vitamin K1 (phylloquinone) is found in green leafy vegetables like kale, spinach, broccoli, and lettuce. Vitamin K2 (menaquinones) comes from some animal foods, fermented foods such as natto, and bacteria in your gut.

Most healthy people get enough vitamin K from a normal diet. Supplements are most often used by people who are deficient or who want to support bone or heart health. Vitamin K is also given as an injection to newborns in hospitals to prevent a rare but serious bleeding problem.

How it works

Vitamin K acts as a helper for an enzyme that "activates" certain proteins in the body. Without enough vitamin K, these proteins cannot do their jobs properly.

In the liver, vitamin K helps create the clotting factors that allow your blood to form clots and stop bleeding. In bones and blood vessels, it helps activate proteins (such as osteocalcin and matrix Gla protein) that are thought to help direct calcium to the right places. Some of the bone and heart-related effects come mainly from laboratory and observational research, so they are not fully proven in people.

Effectiveness

Does Vitamin K work?

Here’s what the research actually shows for each use — graded by Natural Medicines and reviewed by Dr. Brian Staiger, PharmD.

Evidence overview · 32 uses evaluated

4 Effective 1 Possibly effective 1 Leans ineffective 26 Insufficient evidence

How to read these evidence grades

Natural Medicines’ 7-point scale. We show each rating’s label word-for-word.

1EffectiveStrong, consistent evidence it works.

2Likely EffectiveGood evidence, though not yet conclusive.

3Possibly EffectiveSome evidence suggests a benefit.

4Possibly IneffectiveSome evidence it may not help.

5Likely IneffectiveFairly strong evidence it doesn’t help.

6IneffectiveStrong evidence it doesn’t work.

7Insufficient Reliable Evidence to RateToo little research to say either way.

Effective Hemorrhagic disease