Warfarin is an oral anticoagulant that has long been used to treat deep vein thrombosis (DVT) and prevent future clots. It works by blocking vitamin K–dependent clotting factors in the liver, which slows down the blood’s ability to clot. Warfarin requires regular blood tests (INR monitoring) to make sure the dose is safe and effective, since its activity can be affected by diet, other medications, and illness. It is effective for DVT treatment, but newer drugs like apixaban are often preferred because they need less monitoring and have more predictable effects.