Warfarin sodium, C19H15NaO4 is an anticoagulant which acts by inhibiting vitamin K-dependent coagulation factors.

Warfarin is used in various medications for treatment of conditions where blood clotting is undesirable, including many heart conditions, and stroke.

It can also be found as the main active ingredient of Ratsak, a rat poison. The Warfarin causes the rats to internally bleed to death.

A sufficient dose would have a similar effect on humans - it could potentially render a tiny bruise fatal.

The vitamin K dependent coagulation factors that are inhibited by warfarin are, of course, Factor II, Factor VII, Factor IX and Factor X (medical students in Australia remember these as the TV Channel Factors). Warfarin also acts on protein C and protein S (which are anticoagulants).

A vitamin K injection is given to reverse the action of warfarin. Even so, the coagulability of the blood can take up to a few days to come back to normal.

Warfarin is given for mechanical heart valve replacement, deep venous thrombosis (as prophylaxis against pulmonary embolism), in atrial fibrillation and to anyone with a problem with systemic emboli.

Patients currently on warfarin should be warned that as their blood is less coagulable, they need to consult a doctor before undergoing dental surgery or any procedure that may involve bleeding (including catherization). Warfarin is also teratogenic so women of childbearing age should be warned about this. Finally, warfarin has numerous drug interactions with all sorts of other drugs, so a doctor should be consulted before taking or changing any medication, including over the counter drugs and vitamins.

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