Warfarin is an anticoagulant. It is most likely to be the drug popularly referred to as a "blood thinner," yet this is a misnomer, since it does not affect the thickness or viscosity of blood. Instead, it acts on the liver to decrease the quantity of a few key clotting factors in blood that allow blood to clot.
Warfarin is effective and relatively safe for preventing thrombosis and embolism (abnormal formation and migration of blood clots) in many disorders. It was approved for use as a medication in the early 1950s and has remained popular ever since; warfarin is currently the most widely prescribed oral anticoagulant drug in North America.
Despite its effectiveness, treatment with warfarin can be complicated. Many commonly used medications interact with warfarin, as do some foods (particularly plants containing vitamin K), and its activity has to be monitored by blood testing for the international normalized ratio (INR) to ensure an adequate yet safe dose is taken. A high INR predisposes to a higher risk of bleeding, while an INR below the target range indicates that the dose of warfarin is insufficient to protect against thromboembolic events.
The type of anticoagulation (clot formation inhibition) for which warfarin is best suited is that in areas of slowly-running blood, such as in veins and the pooled blood behind artificial and natural valves. Thus, common clinical indications for warfarin use are atrial fibrillation (AF), the presence of artificial heart valves, deep venous thrombosis (DVT), and pulmonary embolism (PE).