Which nutrient interacts with warfarin and requires monitoring of dietary intake?

• A. Vitamin C
• B. Vitamin D
• C. Vitamin K
• D. Vitamin B12

Answer: (C)

Warfarin’s effect hinges on vitamin K levels in the body. It works by inhibiting the recycling of vitamin K, which is needed to activate several clotting factors. Because of that, how much vitamin K you consume from your diet can directly influence how strongly warfarin anticoagulates you.

If you eat more vitamin K than usual, the anticoagulant effect can be reduced, lowering the INR and increasing clot risk. If you eat less vitamin K, the anticoagulant effect can become stronger, raising the INR and increasing bleeding risk. For this reason, dietary vitamin K intake should be kept consistent and monitored through regular INR testing, with dose adjustments as needed.

Other vitamins listed don’t have the same direct, clinically significant interaction with warfarin that requires ongoing dietary monitoring.