Which agent is the reversal drug for warfarin?

Phytonadione, also known as Vitamin K, is the agent used to reverse the effects of warfarin. Warfarin works by inhibiting the vitamin K epoxide reductase complex, which is crucial for the synthesis of vitamin K-dependent clotting factors. By administering phytonadione, the effects of warfarin can be counteracted, allowing for the synthesis of the clotting factors that warfarin inhibits, thus helping to restore normal coagulation.

In clinical situations where rapid reversal of warfarin is required, such as in the case of major bleeding or need for urgent surgery, phytonadione is the standard treatment to restore coagulation. Other treatments, such as prothrombin complex concentrates, can also be utilized in these situations to provide immediate clotting factors, but phytonadione remains the specific reversal agent for the underlying mechanism of warfarin action.

The other agents listed do not serve as reversal agents for warfarin. Protamine sulfate is specifically used to reverse the effects of heparin, idarucizumab reverses the effects of dabigatran, while rivaroxaban is an anticoagulant itself and does not serve any reversal purpose for warfarin.