Macro cross-section of a bone showing the porous honeycomb trabecular structure
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Vitamin K2 for Bones and Blood Thinners: What to Know Before You Start

ByBones After 50 Editorial Team

Short answer: vitamin K2 (as MK-7) may slow bone loss a little in healthy postmenopausal women, but it has not been shown to prevent fractures, and it can interfere with warfarin. If you take a blood thinner, talk to your prescriber before you buy any vitamin K product, including a multivitamin.

Not medical advice. Never start, stop or change a blood thinner or a vitamin K supplement on your own. Your prescriber needs to know what you take so your INR can be monitored.

Why vitamin K matters for bone

Vitamin K helps your body activate osteocalcin, a protein that binds calcium in bone. That is the biological reason K2 supplements are sold for bone health. Our vitamin K2 and bone health guide covers the basics and food sources. This article is about the part that gets less attention: who should be careful.

What the main K2 trial found

A three-year trial gave 244 healthy postmenopausal women either a placebo or 180 mcg of MK-7 a day (Knapen 2013). MK-7 improved vitamin K status and reduced the age-related decline in bone mineral content and density at the lumbar spine and femoral neck, but not at the total hip. It also reduced the loss of vertebral height in the lower thoracic spine. The authors concluded that MK-7 “may help” postmenopausal women prevent bone loss. Three things to keep in mind:

  • It measured bone density and strength indices, not whether fewer women broke a bone.
  • It enrolled healthy women, not people with osteoporosis or on blood thinners.
  • The authors were affiliated with a vitamin K company, which does not invalidate the trial but is worth knowing.

Some popular guides cite an older review of vitamin K2 and osteoporosis (Iwamoto 2004). That review was retracted in 2021 because it relied on sources under investigation for data problems, so it should not be used as evidence.

Why warfarin makes this a safety issue

Warfarin works by blocking the recycling of vitamin K that your clotting factors depend on. More vitamin K can weaken the drug’s effect; sudden changes in intake make your INR swing. That is why anticoagulation clinics ask you to keep your vitamin K intake steady rather than avoid it.

MK-7 is a particular concern because it stays in the blood for a long time. In a study of healthy volunteers, MK-7 had a much longer half-life than vitamin K1, accumulated 7 to 8 fold with prolonged intake, and the authors warned that preparations supplying 50 mcg a day or more of MK-7 “may interfere with oral anticoagulant treatment in a clinically relevant way” (Schurgers 2007). The 180 mcg dose used in the bone trial and sold in many capsules is well above that threshold. The NIH Office of Dietary Supplements also lists warfarin as an interaction for vitamin K supplements.

What about newer blood thinners?

Anticoagulants such as apixaban, rivaroxaban and dabigatran do not work through vitamin K, so they do not have the same dietary interaction. Do not assume this applies to you without asking: which drug you take, and why, is a question for your prescriber or pharmacist.

A practical checklist

  1. If you take warfarin: do not start K2 on your own. If your doctor agrees, they will usually want to check your INR more often when you start and stop.
  2. Check the label. Many multivitamins and “bone support” blends contain vitamin K1 or K2 alongside calcium and D.
  3. Food first. Leafy greens, natto and some cheeses provide vitamin K. If you are on warfarin, keep the amount consistent week to week.
  4. Remember what bone needs most. Loading exercise, enough protein, calcium and vitamin D status, and falls prevention have far stronger evidence than K2. See our prevention plan and what trials show on calcium supplements.

References

  • Knapen MH, et al. Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporos Int 2013;24:2499-507. PubMed
  • Schurgers LJ, et al. Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7. Blood 2007;109:3279-83. PubMed
  • Iwamoto J, et al. Effects of vitamin K2 on osteoporosis (retracted). Curr Pharm Des 2004;10:2557-76; retraction 2021;27:2325. PubMed
  • NIH Office of Dietary Supplements. Vitamin K fact sheet for health professionals. ods.od.nih.gov

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