Does vitamin K interact with any medications?

People taking warfarin have to keep their vitamin K intake steady, because sudden changes make the drug work more or less than intended.

What this means for you

If you take warfarin, this is one of the few pages on this site that should change what you do.

Not for everyone. Vitamin K is not only in greens. Multivitamins, some meal-replacement drinks and green powders contain it, often without saying so on the front of the pack. Starting or stopping one of those is the same kind of change as switching your diet, and it is the version people forget to mention at the clinic.

The instruction people usually hear is "avoid greens", and it is the wrong one. Cutting kale and spinach out does not make the drug safer, it just moves your baseline, and the next salad then swings it back. What your clinic actually needs is a steady intake, week after week, at whatever level suits you.

So pick a pattern you can keep. If you eat greens most days, keep eating greens most days. If you rarely do, that is also fine as a baseline. What causes trouble is the change: a sudden green juice habit, a week of restaurant food with none, a new supplement that contains vitamin K without saying so on the front.

Tell whoever manages your INR when your eating changes, including when you start or stop a multivitamin. That single sentence is more useful than any food list.

If you take medicine for blood sugar

This one is newer and it belongs on the same page for the same reason. Eight randomised trials in adults with prediabetes or type 2 diabetes were pooled. Vitamin K lowered HbA1c by a full percentage point, and moved fasting glucose and insulin resistance a little. Fasting insulin itself did not budge.

Not for everyone. A change of one percentage point in HbA1c is a large one to add on top of medication without telling anybody. If you are on treatment for blood sugar, mention a vitamin K supplement to whoever adjusts your doses, whether you are starting it or stopping it. Eight trials is a small base to stand on, and none of this is a reason to take vitamin K for diabetes in the first place.

Warfarin remains the one that changes what you do this week. Everything else on this page is worth a sentence at your next appointment.

The bone claim, at its 2026 size

People reach for vitamin K supplements mostly because of bones, so it is worth having the current number rather than the one that got the claim started.

Unsettled. The observational fracture finding was pooled again in 2026 and came out as a trend rather than a result. Overall fractures ran at a risk ratio of 0.83 with a confidence interval from 0.68 to 1.01, which touches no effect. Hip fractures ran at 0.76 with an interval from 0.56 to 1.02, which also touches it. The direction has been consistent across a great deal of work and the significance keeps failing to arrive, and both halves of that belong in the same sentence. This is observational data about what people eat, which is a weaker thing than a trial of a supplement, and the supplement trials have been less encouraging still.

Established. One detail inside that same pooling is useful whatever you make of the rest. Fracture risk fell with vitamin K intake only up to about 120 micrograms a day, which is the reference intake itself, and the authors describe the association as U-shaped around that level. More was not better, and past a point more looked worse. That is an argument against high-dose K2 products built from the very data used to sell them.

The review that put the question bluntly, asking whether vitamin K for bones is myth or reality, concluded that the findings are conflicting and unclear. The biology is genuinely appealing. Vitamin K carboxylates osteocalcin, which governs how bone takes up mineral, and it appears to help osteoblasts mature into osteocytes and to hold back the cells that break bone down. Appealing biology with conflicting trials is a pattern worth recognising, because vitamin E is the same story with a longer record.

Not for everyone. None of this changes the warfarin advice above, which remains the most useful thing on this page. If you are on warfarin, the point is steadiness rather than avoidance, and a supplement is a change in intake like any other. Discuss it before starting rather than after.

Written from the evidence cards and the data on this page, then checked back against the sources. Bioma Learn has no human medical reviewer at this time and we say so rather than invent one. Information, not medical advice. How we verify.

Full guide: Vitamin K: the nutrient whose most useful fact is about a medicine