Warfarin

Warfarin (sometimes sold as Coumadin) is a powerful water-soluble compound. It prevents blood from clotting. It does that by reducing the production of factors by the liver that promote clotting. It is called an "anticoagulant agent". It is a powerful rat poison. In concentrated form, it causes bleeding of internal organs which leads to death.
It is also used, in very low doses, to prevent thrombosis in humans. This is the local coagulation or clotting of the blood in the circulatory system. In such cases, it makes heart attacks, strokes, and pulmonary embolism less likely. The degree of anticoagulation is monitored by blood tests.