Vitamin D3 + K2 and blood thinners: the interaction

With a combined vitamin D3 and K2 product there is exactly one interaction that genuinely matters, and it concerns a single group of medicines: the coumarin-type anticoagulants, that is phenprocoumon (Marcumar, Falithrom), warfarin and acenocoumarol. These drugs intervene in vitamin K metabolism; vitamin K taken in from outside weakens their effect. The newer direct oral anticoagulants such as apixaban or rivaroxaban, and antiplatelet drugs such as acetylsalicylic acid, work through entirely different routes — vitamin K passes them by. The vitamin D in the same product plays no part in blood clotting at all.

The decisive question is therefore not "am I allowed to take vitamin K2" but "which medicine am I taking". Everything else follows from that.

Which medicine — everything depends on it

Drug group	Examples	How it works	Does vitamin K matter?
Vitamin K antagonists (coumarins)	phenprocoumon, warfarin, acenocoumarol	block the recycling of vitamin K in the liver	yes, directly
Direct oral anticoagulants (DOACs)	apixaban, edoxaban, rivaroxaban	direct, reversible factor Xa inhibitors	no
Direct oral anticoagulants (DOACs)	dabigatran	direct, reversible thrombin inhibitor	no
Antiplatelet drugs	acetylsalicylic acid, clopidogrel	act on the platelets	no

The mechanisms of the four DOACs are described this way in the guidance "Wirkstoff AKTUELL" issued by the Drug Commission of the German Medical Association (issue 4/2017). What counts is what the right-hand column says and what it does not: it answers the vitamin K question, not the question of whether a product suits a particular person. Each of these medicines has its own warnings in its summary of product characteristics — just not this one.

In Germany the coumarin group is anything but a marginal case. The Federal Institute for Risk Assessment (BfR) cites a pharmaceutical report according to which, of roughly 5.5 million packs of oral anticoagulants dispensed in 2012, close to 76 per cent were vitamin K antagonists. That share has since fallen in favour of the DOACs, but the group remains large — and it consists mainly of older people, that is precisely those who also reach for vitamin D.

Why coumarins and vitamin K cancel each other out

Four clotting factors — II, VII, IX and X — only become functional in the liver once an enzyme finishes them chemically, consuming vitamin K in the process. The vitamin K is used up and then reprocessed so that it is available again. It is exactly this reprocessing that the coumarins block. The body then has vitamin K but can no longer cycle it; the factors stay unfinished and the blood clots more slowly.

If vitamin K is now supplied from outside, the pool is topped up directly and the blockade is partly bypassed. The INR value — the standardised measure of clotting time — falls, and the therapy works more weakly than it was set up to. The reverse holds just as well: in someone who has taken vitamin K for weeks and suddenly stops, the INR rises. That is why the recommendation of the EU Scientific Committee on Food, which the BfR adopts, is not simply a prohibition but a recommendation of consistency: those affected should keep their dietary vitamin K intake "as constant as possible and use vitamin K-containing food supplements only under medical supervision".

This also explains why the usual advice to "just leave a few hours between them" does not apply here. With minerals that compete for the same transport routes in the gut, a few hours' gap genuinely helps. Vitamin K2 in the MK-7 form, by contrast, has a half-life of around 68 hours; with daily intake serum levels climb to a multiple of baseline and stay stable for days. There is no time of day at which it is absent. More on this difference is in Vitamin K2: MK-7 or MK-4.

How little K2 is already enough

The amounts involved are smaller than most people assume. Two studies measured the dose-response relationship systematically: volunteers were stabilised on acenocoumarol at an INR of 2.0 and then received rising amounts of vitamin K over several weeks.

Amount per day	What was observed
150 µg vitamin K1	threshold for a statistically detectable INR reduction; clinically relevant in 25 % of participants
10 µg vitamin K2 (MK-7)	clinically relevant INR reduction in around 40 % of participants
20 µg vitamin K2 (MK-7)	clinically relevant INR reduction in around 60 % of participants
45 µg vitamin K2 (MK-7)	threshold for the statistically detectable reduction; INR about 40 % lower in the group mean

The K1 figures come from Schurgers and colleagues (Blood 104, 2004), the K2 figures from Theuwissen and colleagues (Journal of Thrombosis and Haemostasis 11, 2013). From the comparison of the two thresholds — 150 µg against 45 µg — the BfR derives that vitamin K2 counteracts coumarin therapy about 3.5 times as strongly as vitamin K1. The authors of the K2 study conclude from this that MK-7 products should be avoided during treatment with vitamin K antagonists.

From the same data the BfR derived its proposed maximum levels for food supplements: with an uncertainty factor of around 2 it arrives at 80 µg vitamin K1 and 25 µg vitamin K2 respectively per recommended daily portion of a product. Alongside this it recommends the following warning on the pack: "People taking anticoagulant medicines should seek medical advice before consuming food supplements containing vitamin K." Both are a proposal, not law in force — there is no statutory maximum level for vitamin K in Germany, and a Tolerable Upper Intake Level could never be derived for vitamin K for lack of data.

What is actually in a D3 + K2 product

Commercially available combination products sit well above every threshold named — this is not a quirk of individual brands but the norm in this product category:

Product	K2 per tablet or capsule	Ratio to the BfR proposal (25 µg)
Vitamin D3 4,000 IU + K2 – 365 tablets	200 µg MK-7	8 times
Liposomal D3 5,000 IU + K2 – 180 capsules	100 µg MK-7	4 times
Vitamin D3 20,000 IU + K2 – 360 tablets	200 µg MK-7	8 times

For context: from ordinary food, participants in the EPIC-Heidelberg study took in a median of 93.6 µg vitamin K1 and 34.7 µg vitamin K2 per day. A single combination tablet therefore delivers more MK-7 than a whole day of eating — and MK-7 is the more potent of the two forms. How the microgram amounts are distributed across the strengths, and why they are not tied to the vitamin D content, is set out in Vitamin D3 and K2: is there a correct ratio?.

With a depot tablet taken only every ten or twenty days, the amount spreads across more days on paper. That does not make the intake steady: it arrives in bursts, and that is exactly the unfavourable case under an established coumarin therapy.

What this means in practice

Anyone taking phenprocoumon, warfarin or acenocoumarol should not start a vitamin K-containing product on their own initiative — and should not abruptly stop one already started either. Both shift the INR, only in opposite directions. The decision, and INR monitoring where needed, belong with the treating practice; that is the core of the BfR warning.

Anyone who needs vitamin D but does not want vitamin K has a simple solution: a plain D3 product. In a combination tablet the two substances cannot be separated — you take both or neither. That is why we also carry vitamin D3 without K2, for example as Vitamin D3 4,000 IU – 365 tablets or as Vitamin D3 10,000 IU vegan – one-year supply. All strengths side by side are in the vitamin D3 category, the combinations in the vitamin K2 category.

For anyone on a DOAC or acetylsalicylic acid, this calculation does not arise. The question of the right vitamin D strength remains open regardless — the common steps are worked through in Vitamin D3 + K2 dosage: 1,000 or 5,000 IU?.

Before planned surgery or a dental procedure, a vitamin K-containing product belongs on the list of things you mention anyway — simply because it affects the same lever as the anticoagulation itself.

And the vitamin D?

Vitamin D takes no part in the clotting cascade; the interaction with anticoagulants comes exclusively from the vitamin K. For vitamin D, on the other hand — unlike vitamin K — an upper limit does exist: EFSA gives adults a tolerable total daily intake of 100 µg, corresponding to 4,000 IU. It refers to long-term intake from all sources combined and is not a threshold beyond which something happens immediately. Which raw materials sit behind the two vitamins is covered in Vegan vitamin D3 with K2.

A second point belongs here for completeness: vitamin D increases calcium absorption from the gut. Anyone taking medicines that themselves affect calcium balance is well advised to discuss a permanently high vitamin D intake with their doctor as well. This has nothing to do with blood clotting.

What may appear on the tin

The authorised health claims are listed in Regulation (EU) No 432/2012:

• Vitamin K contributes to normal blood clotting.
• Vitamin K contributes to the maintenance of normal bones.
• Vitamin D contributes to the maintenance of normal bones.
• Vitamin D contributes to the normal function of the immune system.

The first of these claims captures the situation exactly: vitamin K is part of normal clotting. Anyone deliberately keeping that clotting damped down with medication therefore has a different interest in the vitamin than everyone else.

Food supplements are not a substitute for a balanced and varied diet and a healthy lifestyle.

Common questions

I take Marcumar — may I take a D3 + K2 product?

That is decided by the treating practice, not by the pack. Phenprocoumon is a vitamin K antagonist, and the usual 100 to 200 µg of MK-7 sit far above the amount at which an INR reduction was measurable in studies. For such products the BfR explicitly recommends the note to seek medical advice before consumption. Anyone who only wants the vitamin D sidesteps the question with a plain D3 product.

Does it help to take the tablet at a different time of day?

No. MK-7 has a half-life of about 68 hours and accumulates in serum with regular intake — a gap of a few hours changes nothing. Spacing doses out is a sensible measure for nutrients that obstruct each other in the gut; here the effect sits in liver metabolism.

Do I have to give up kale and spinach while on anticoagulants?

No — and this is a widespread misunderstanding. The EU Scientific Committee on Food recommends keeping dietary vitamin K intake constant, not minimising it. Several studies show that a steady, adequate vitamin K intake tends to make it easier to settle on the optimal coumarin dose. Fluctuation is the problem, not the amount as such.

I take apixaban or rivaroxaban — does this concern me?

These drugs inhibit clotting factor Xa directly and do not need the vitamin K cycle to do so. There is therefore no INR to be adjusted here, and no interaction via vitamin K. The BfR nevertheless phrases its warning generally for "anticoagulant medicines" — a brief mention at the practice costs nothing.

Does this apply to acetylsalicylic acid too?

Acetylsalicylic acid inhibits the platelets and is not a vitamin K antagonist; the mechanism described here does not apply. Anyone taking acetylsalicylic acid together with a coumarin is in a medically supervised situation anyway, in which an additional vitamin K product belongs in the conversation.

Sources: BfR, "Höchstmengenvorschläge für Vitamin K in Lebensmitteln inklusive Nahrungsergänzungsmitteln" (opinion, sections 1, 2.1 and 2.3, referring to SCF 2003 and the Barmer-GEK-Arzneimittelreport 2014); Schurgers LJ et al., "Effect of vitamin K intake on the stability of oral anticoagulant treatment: dose-response relationships in healthy subjects", Blood 104(9), 2004 (PubMed 15231565); Theuwissen E et al., "Effect of low-dose supplements of menaquinone-7 (vitamin K2) on the stability of oral anticoagulant treatment: dose-response relationship in healthy volunteers", Journal of Thrombosis and Haemostasis 11(6), 2013 (PubMed 23530987); Arzneimittelkommission der deutschen Ärzteschaft, "Wirkstoff AKTUELL" 4/2017, direct oral anticoagulants, table 1; Nimptsch K et al., EPIC-Heidelberg (Am J Clin Nutr 87, 2008); Schurgers LJ et al., "Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7", Blood 109(8), 2007; EFSA, Scientific Opinion on the Tolerable Upper Intake Level of vitamin D (2012) and Dietary Reference Values for vitamin K (2017); Regulation (EU) No 432/2012.