Vitamin K
Vitamin K, the generic name for a family of compounds, is a fat-soluble vitamin that is naturally present in some foods. It functions as a coenzyme for vitamin K-dependent carboxylase, which is required for the synthesis of proteins involved in processes such as blood clotting (hemostasis) and bone metabolism. Vitamin K transport across the placenta is poor, increasing the risk of vitamin K deficiency in newborn babies. People with malabsorption disorders might need monitoring of vitamin K status and, in some cases, vitamin K supplementation.
Where to get it
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Active compounds & mechanisms
Key compounds: Phytonadione, MK-4
Research & evidence
Strong support Some support Weak or no supportColours show how well the sources back each point. The reason is next to each one.
NIH ODS reports that MK-4 supplementation significantly reduced the rates of hip fractures, vertebral fractures, and all nonvertebral fractures. It also says it is not clear whether vitamin K supplementation affects the risk of osteoporosis, and other research has found that vitamin K supplementation has no effect on bone mineral density in older men or women.
Mixed evidenceSome support. Results are mixed
Study details
NIH ODS says: "MK-4 supplementation significantly reduced the rates of hip fractures, vertebral fractures, and all nonvertebral fractures." "Despite this, it is not clear whether vitamin K supplementation affects the risk of osteoporosis." "it is unclear whether supplementation with any form of vitamin K reduces the risk of osteoporosis" "have found that vitamin K supplementation has no effect on bone mineral density in older men or women"
Source: NIH ODS, Vitamin K — Health Professional Fact Sheet (Updated October 22, 2026), Vitamin K and Health: Osteoporosis View source
The role of the different forms of vitamin K on vascular calcification and the risk of coronary heart disease is unclear, and the results of clinical trials have been inconclusive. NIH ODS also notes there was no significant difference in coronary artery calcification between the treatment and control groups.
Limited evidenceSome support. Reviewers rated the certainty low
Study details
NIH ODS says: "the role of the different forms of vitamin K on vascular calcification and the risk of coronary heart disease is unclear" "the results of clinical trials have been inconclusive" "There was no significant difference in coronary artery calcification between the treatment and control groups."
Source: NIH ODS, Vitamin K — Health Professional Fact Sheet (Updated October 22, 2026), Vitamin K and Health: Coronary heart disease View source
Dosage
| Form | Amount |
|---|---|
| Any form | NIH ODS gives Adequate Intakes (AIs) rather than RDAs for vitamin K: 90 to 120 mcg a day for adults. No upper limit has been set for vitamin K because it has a low potential for toxicity. |
Side effects
Well documented Reported (case reports, anecdotal or one study) None reportedThe circle shows how well documented an effect is, not how serious it is. A rarely reported effect can still be serious, so always read ‘When to stop’.
| Effect | Frequency | When to stop |
|---|---|---|
| Interference with blood-thinning medications (particularly warfarin)Well documentedListed plainly by NIH, MSK or LiverTox | Clinically significant at any dose for warfarin users | Do not supplement without consulting a doctor if you take warfarin or other blood thinners. |
Safety & interactions
Safety
Vitamin K does not have a Tolerable Upper Intake Level because it has a low potential for toxicity. No adverse effects associated with vitamin K consumption from food or supplements have been reported in humans or animals. Vitamin K can have a serious and potentially dangerous interaction with anticoagulants such as warfarin (Coumadin).
Contraindications
Vitamin K can have a serious and potentially dangerous interaction with anticoagulants such as warfarin (Coumadin). If you take an anticoagulant, see Drug interactions.
Drug interactions
NIH ODS says vitamin K can have a serious and potentially dangerous interaction with anticoagulants such as warfarin (Coumadin).
NIH ODS also names phenprocoumon, acenocoumarol and tioclomarol, which are commonly used in some European countries, in this anticoagulant interaction.
NIH ODS says an effect noted in its vitamin K drug interactions section might be more pronounced with cephalosporin antibiotics, such as cefoperazone (Cefobid). Our sources don't describe the effect.
NIH ODS lists bile acid sequestrants, such as cholestyramine (Questran) and colestipol (Colestid), which are used to reduce cholesterol levels, in its vitamin K drug interactions section. Our sources don't describe the effect.
NIH ODS lists orlistat, a weight-loss drug available both over the counter (Alli) and by prescription (Xenical), in its vitamin K drug interactions section. Our sources don't describe the effect.
Established: stated plainly by an authoritative source. Documented: supported by case reports or trial data. Theoretical: based on mechanism, animal data, or a cautious listing. Always check with your pharmacist or prescriber.
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