Warfarin (Coumadin) treats blood clots, such as those in deep vein thrombosis and pulmonary embolism.
When taken, patients’ international normalized ratio (INR) test should be monitored on a regular basis.
Vitamin K works against warfarin and can be used as an antagonist for it.

Explanation

Vitamin K blocks the warfarin effects and thus causing lower INRs.
The lower the INR, the less time it takes for the blood to clot.
A low INR means the warfarin dose is not working well enough to prevent dangerous blood clots.
Vitamin K, can be prescribed in various dosage forms, such as oral, to help stop the affect of warfarin, if needed
Foods that contain too much vitamin K should be avoided when taking warfarin, which include green leafy vegetables.

Reference