AVOID starting Vitamin K2 supplementation without specialist review if you are on warfarin or another coumarin anticoagulant. Vitamin K antagonises the mechanism of these drugs and can reduce INR unpredictably.
Mechanism
Warfarin works by inhibiting vitamin K epoxide reductase, the enzyme that regenerates active vitamin K used to gamma-carboxylate clotting factors II, VII, IX and X. Vitamin K (including the K2 / MK-7 form sold in supplements) provides substrate for the residual pathway and can therefore overcome the anticoagulant effect to varying degrees, reducing INR and increasing thrombotic risk.
Practical advice
If you are on warfarin and considering Vitamin K2 supplementation, do not start without discussing with the clinician managing your anticoagulation. If you are already taking both and have not declared this, get an INR check and tell your anticoagulant clinic.