What should patients on warfarin know about vitamin K?

• A. Maintain consistent intake
• B. Avoid all foods with vitamin K
• C. Increase vitamin K intake during therapy
• D. Vitamin K has no effect

Answer: (C)

Warfarin activity hinges on vitamin K because warfarin works by blocking the recycling of vitamin K needed to activate clotting factors. If a patient’s vitamin K intake changes from day to day, the level of active clotting factors can vary, causing the INR to swing. That makes it harder to keep anticoagulation in a safe, steady range.

So the best instruction is to maintain a consistent amount of vitamin K in the diet. This helps keep the INR stable and reduces the risk of either bleeding (if the INR is too high) or clotting (if it’s too low). It’s not necessary to eliminate vitamin K foods entirely, and increasing vitamin K during therapy generally counteracts warfarin’s effect rather than helping it; any dietary changes should be discussed with a clinician. Vitamin K does have an effect on warfarin.