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In addition to vitamin K playing a role in bone health, it helps make proteins needed for clotting. Anticoagulant drugs like warfarin (Coumadin and Jantoven) work by blocking the production of those clotting proteins, helping to "thin" the blood and prevent clotting. Not surprisingly, increasing your intake of vitamin K long-term may require you to slightly increase the dose of these blood thinners and, similarly, people taking these blood thinners are advised to avoid wide fluctuations in their vitamin K intake.

Although low-dose vitamin K supplementation (100 to 200 mcg of vitamin K1 daily) was previously recommended to help stabilize fluctuations in blood clotting in people taking warfarin, experts now recommend against doing this (for more details, see the What It Does section of the Vitamin K Supplements Review).

Because vitamin K2 from food is associated with a reduced risk of coronary calcification and mortality from coronary heart disease, some people assume it is safer to take this form with warfarin than vitamin K1. In addition, the MK-7 form of vitamin K2 has a longer half-life than K1, which may account for better stability of INR values (a clotting measurement) for people on anticoagulant therapy. However, MK-7 has been shown to interfere with anticoagulant therapy, and at much lower doses than vitamin K1. For more details, see the Concerns and Cautions section of the Vitamin K Supplements Review.

(Be aware that CoQ10 is chemically similar to vitamin K2 and may also decrease the effects of warfarin, although the evidence for this is mixed. See Concerns and Cautions in the CoQ10 Supplements Review).

Unlike warfarin, some newer blood thinning drugs, such as dabigatran (Pradaxa), apixaban (Eliquis) and rivaroxaban (Xarelto) are not thought to be affected by vitamin K intake. However, some of these drugs can be affected by other supplements (for more details, see our answer to the question: Are there supplements I should avoid when taking apixaban (Eliquis) or similar blood thinner drugs?).

Another way to prevent clots is by taking aspirin or related drugs, but these work in a different way - preventing platelets from clumping together to form clots. There does not appear to be evidence of vitamin K interacting with antiplatelet drugs.

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July 13, 2017

I'm on warfarin & tried MK-7 for my osteoporosis. The results on my next DEXA scan were – mixed: some areas deteriorated, some improved. I'm currently trying isometric exercises (various poses), since I heard that exercise doesn't build bone unless you hold the stress position for 8 seconds. (But I'm also continuing MK-7.) I heard the 8-second rule on Peoples Pharmacy from guest Loren Fishman M.D. So I bought his book (but I don't follow the yoga approach closely).

August 14, 2016

I have exactly the he same positive experience with a high dose vitamin K complex with MK-7. Yes, my dosage needed to be raised but once adjusted, my INR stays very stable. Unfortunately most nurses and docs don't know about this. I did this experiment with myself and it WORKED! :)

October 8, 2015

When I first started taking Coumadin, I found it difficult to keep my INR stable. It would fluctuate widely, depending on my intake of greens. Also, I had read that Coumadin promotes artery calcification. Instead of going to bones and teeth, calcium gets deposited in soft tissues (like arteries). Then I found research indicating that Vitamin K2 might help to make sure that calcium went to bones and teeth and also help to stabilize INR. I don't know whether it's helping my bones or not, but I've found that taking Vitamin K2 MK-7 (menaquinone-7) instead of K has resulted in a very stable INR. Apparently, this is because K2 MK-7 has a long half-life in the blood. I did have to compensate by taking more Coumadin, but once I did that, I could depend on very stable levels regardless of variations in my intake of Vitamin K-rich vegetables.

October 7, 2015

My wife was on Coumadin for years due to increased clotting from atrial fibrillation (clots forming in vibrating but non-beating atria). We gave her a fixed amount of Vitamin K every day, 100 mcg, by either having a salad we had computed this amount in or 1/8 cup of cooked and frozen spinach with same 100 mcg. Thus we were able to control her INR to a 1/2 point range (rather than the 1 point range recommmended between 2.0 and 3.0. Then we put her range where we thought it best (with Dr's advance approval), near low end if her AF was controlled my Amiodarone, or nearer high end if she was having bouts of AF (this showed in her by high pulse values).

So, common advice to "avoid Vitamin K" is not best thing to do. Fortunately our cardiologist told us right off that was NOT true. Consistent K could be compensated for by increasing Coumadin dose from perhaps 2.5 mg/day to 3.75 mg some days, and fine tuned by adjusting amount of K in diet.

October 7, 2015

I would clarify that it is Vitamin K2 you are asking about? That is the form of K that is most associated with bone health and maintenance.

October 7, 2015

My daughter is geneically prone to blood clots and takes curcumin liquid from Natural News. She had a clot, they put her on Cereltoe beause of change of life, she almost bleed to death. Had to spend weekend getting blood transfusions. I have no interest in any of these products, just a caution to woman that may be in same situation. Her doctor gave her a gene test, and it was from her fathers side of family.

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