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.