Ingredient
Vitamin K
A vitamin whose ordinary job is the thing one common medicine is designed to prevent, which makes it the clearest interaction on a supplement shelf.
- Also on labels as
- Phylloquinone, Vitamin K1, Menaquinone, Vitamin K2, MK-7, Phytonadione
- Found in
- supplements, food
Part of Medicines · Animal ingredients
Most vitamins on this site are interesting because of where they come from. This one is interesting because of what it collides with.
What it does
The NHS puts it in one line: vitamin K “is a group of vitamins that the body needs for blood clotting, helping wounds to heal”. There is also some evidence it helps keep bones healthy.
The requirement is unusual in being expressed per kilogram: adults need “approximately 1 microgram a day of vitamin K for each kilogram of their body weight”. So a 65 kilogram person needs about 65 micrograms and a 75 kilogram person about 75.
It is found in “green leafy vegetables – such as broccoli and spinach”, in vegetable oils and cereal grains, with small amounts in meat and dairy.
The collision
Warfarin is an anticoagulant that works by interfering with vitamin K. That is its mechanism, not a side effect.
So a vitamin K supplement is not merely something to be careful about alongside warfarin, it works directly against what the medicine is doing. NHS guidance is unambiguous: “Do not take the herbal remedy St John’s wort or vitamin K supplements while you’re taking warfarin.”
That is one of very few flat instructions in this whole area, and it is worth treating as one.
Food is a different question, and the answer surprises people
The instinct on hearing this is to stop eating spinach. That is the wrong response and the guidance says so.
The NHS records that foods high in vitamin K “can affect how warfarin works” and then: “You can still eat these foods, but talk to your warfarin clinic”.
The reason is that warfarin dosing is adjusted to a person’s actual intake. What causes trouble is change, not amount. Someone who eats a lot of greens has a dose calibrated for that; the risk arrives when they suddenly stop, or suddenly start a green smoothie habit.
Consistency rather than avoidance. That is a genuinely counter-intuitive rule and it is why the clinic, which holds your numbers, is the right place for the conversation.
K1 and K2 are not the same on a label
Two forms turn up and the distinction matters for where they come from.
K1, phylloquinone, is the plant form and the one in leafy greens and vegetable oils. This is most of dietary intake.
K2, menaquinone, comes from fermented foods and animal sources, and gut bacteria make some. Supplements sold as MK-7 are usually fermented, often from natto, and are therefore generally suitable for a plant-based diet, but the form does not tell you that on its own and a supplement using an animal-derived K2 will not say so either.
That is the same origin problem the rest of this site keeps meeting, set out in the list of ingredients whose source the label hides.
Where it appears without being asked for
In multivitamins, in bone health formulas alongside vitamin D and calcium, and increasingly in vitamin D products because the two are marketed together.
That is the configuration most likely to catch someone out: a person on warfarin buys a bone supplement for an unrelated reason and does not register that vitamin K is in it. Reading the panel on a combination product is the whole defence, and it is the argument of the page on reading labels when you take a prescription.
Sources
- Vitamin K National Health Service, United Kingdom · nhs.uk States that vitamin K is a group of vitamins the body needs for blood clotting, that it is found in green leafy vegetables, vegetable oils and cereal grains, and that adults need approximately 1 microgram a day per kilogram of body weight.
- Warfarin National Health Service, United Kingdom · nhs.uk States not to take St John's wort or vitamin K supplements while taking warfarin, and that foods high in vitamin K such as broccoli, spinach and other green leafy vegetables can affect how warfarin works.