Vitamin K
Brodifacoum inhibits the enzyme vitamin K epoxide reductase, which is needed for the reconstitution of the vitamin K in its cycle from vitamin K-epoxide, so brodifacoum steadily decreases the level of active vitamin K in the blood. Vitamin K is required for the synthesis of important substances including prothrombin, which is involved in blood clotting. This disruption becomes increasingly severe until the blood effectively loses any ability to clot.