Warfarin is an oral blood thinner (anticoagulant) that can be used to treat pulmonary embolism (PE). It works by interfering with vitamin K–dependent clotting factors, which reduces the blood’s ability to form new clots and prevents existing ones from getting larger. Warfarin requires regular blood tests (INR checks) to ensure the dose is safe and effective, since its effect can be influenced by diet, other medications, and illness. Because it takes a few days to become effective, patients are usually started on heparin or low molecular weight heparin at the same time until warfarin reaches therapeutic levels. While effective, the main risk is bleeding, so careful monitoring is essential.