Vitamin K2

Is vitamin K2 safe?

For most adults, yes. Vitamin K2 has no known toxicity, and neither the US National Academies nor the European Food Safety Authority has set an upper limit. The risk is concentrated in one group: people taking warfarin or a related drug such as acenocoumarol or phenprocoumon. For them a normal MK-7 capsule is enough to weaken the drug and raise the chance of a clot.

The second problem is the bottle. Laboratory tests in Europe have found that many MK-7 supplements contain less of the active form than the label says.

Why K2 and warfarin do not mix

Warfarin works by blocking the recycling of vitamin K, which the liver needs to make clotting factors. Extra vitamin K pushes against it. That is true of all vitamin K, but MK-7 is a stronger opponent than the K1 in leafy greens because it stays in the blood for days. A 2007 study in Blood found that MK-7 built up to levels 7 to 8 times higher than K1 during daily use, and warned that 50 mcg a day or more could interfere with anticoagulant treatment.

A 2013 Dutch trial tested lower doses. Eighteen healthy volunteers were put on acenocoumarol, a warfarin-type drug, and then given rising doses of MK-7 for six weeks. At 45 mcg a day, which the authors called the usual retail dose, the group's INR fell by about 40%. Two haematologists who independently reviewed the results judged that 10 and 20 mcg a day caused a clinically relevant drop in at least 40% and 60% of people. The authors concluded that MK-7 supplements should be avoided by people on these drugs.

The Linus Pauling Institute puts the contrast plainly: up to 100 mcg of K1 a day is considered safe with warfarin, but 10 to 20 mcg of MK-7 may undermine it. The usual advice for warfarin users is a steady, normal vitamin K intake (90 to 120 mcg a day), not avoidance, and no sudden changes.

Newer blood thinners such as apixaban and rivaroxaban do not work through vitamin K, so this interaction does not apply to them. Cholestyramine, orlistat and mineral oil can reduce how much vitamin K is absorbed, which matters little for most people.

MK-4 or MK-7: does the form change the risk?

The two forms behave very differently, which is why the doses on the shelf look so different.

MK-4 (menatetrenone) is used in Japan to treat osteoporosis, according to the Linus Pauling Institute. A 2014 Chinese trial gave 45 mg a day to 236 women for a year and reported it as safe. A 2006 meta-analysis in Archives of Internal Medicine found fewer fractures with MK-4, but all seven trials with fracture data were Japanese. For MK-7, a three-year trial at 375 mcg a day found no effect on bone density.

Does it keep calcium out of the arteries?

This is the main marketing claim, usually on vitamin D and K2 combination products. K2 activates matrix Gla protein, which helps stop calcium from building up in blood vessels, and observational studies link higher K2 intake to less heart disease.

Trials have not backed this up. In the Danish AVADEC trial, 365 older men with calcified heart valves took 720 mcg of MK-7 plus vitamin D, or placebo, for two years. Valve calcification progressed at the same rate in both groups. A small 2019 Dutch trial of 360 mcg a day in people with type 2 diabetes and heart disease also found no reduction in artery calcification after six months, and reported no adverse events. None of these trials reported safety problems, which is useful evidence at high doses.

How good are the products?

MK-7 is a fragile molecule. It exists in several geometric forms, and only the all-trans form is active. A 2018 study from Poland measured the forms in dietary supplements and found that in most of them the active form was below the declared amount, ranging from 5.5 to 49 mcg per pill. One product contained none at all, and in some the inactive forms outnumbered the active one by up to 3.7 times. A 2019 Italian study found that less pure MK-7 raw material broke down faster in storage, including in mixtures with common excipients, which may explain why finished products often miss their label.

We found no FDA recalls of K2 products. That reflects how rarely regulators test them, not proof of accuracy.

Who should be careful

• People on warfarin, acenocoumarol or phenprocoumon: talk to your anticoagulation clinic before starting, stopping or changing any product with K2, including multivitamins and vitamin D combinations.
• Anyone having surgery who has been told to adjust a blood thinner: mention every supplement.
• Pregnant women: normal food intake is fine; there is little data on high-dose supplements, so ask your doctor or pharmacist.
• Buyers in general: if you take K2 with vitamin D, prefer a product with independent testing, and do not assume it protects your arteries.

How we scored it

Dose safety (92), organ toxicity (92) and regulatory record (92) are strong: no known toxic dose, no organ injury reports and no bans. Interactions (62) are held back by a single but serious clash with warfarin-type drugs at ordinary retail doses. Product quality (55) is the weakest factor, because independent tests found most MK-7 products short of their stated active content.