WARFARIN

Warfarin (coumadin) is an anticoagulant drug, which means it works by preventing the blood from clotting (secondary homeostasis), it's considered as vitamin K reductase inhibitor, so what is vitamin K reductase to begin with! it's basically an enzyme that is responsible to return vitamin K from it's "INACTIVE" form to the "ACTIVE" one, because vitamin K works in the clotting process by converting the clotting factor "X, IX, VII, II" from their "INACTIVE" form to the "ACTIVE" one, so when we inhibit the vitamin K reductase enzyme, the consumed vitamin K will not return to it's active form, and that will decrease the clotting factors which are dependent on it (X, IX, VII, II), in other words the clotting mechanism will stop.

It's worth mentioning that the clotting mechanism involves three pathways, the extrinsic, the intrinsic, and the common pathway, the extrinsic involves factor VII, the intrinsic involves factors XII, XI, IX, VIII, and the common involves factors X, II, the intrinsic and the extrinsic pathways stimulates the common pathway which in turn converts the the unstable fibrinogen to the stable fibrin, and as you saw earlier warfarin works by inhibiting the factors X, IX, VII, II, which means it will affect all of the three pathways, and ultimately will prevent clotting.

We should not begin with warfarin alone! but WHY? for the first three days we should start the patient on heparin, then we should discontinue heparin and continue with warfarin, once again but WHY? Well, it's because of two main reasons...
The first reason is that warfarin will prevent the formation of new activated clotting factors, but it will NOT have any effect on the ALREADY activated clotting factors.
The second reason reason is that it will cause hypercoagulable state, because vitamin K is also responsible of converting PROTEIN C from the inactive to the active form, and the function of protein C in the body is to prevent clotting, so when it's absent, it will lead to clotting and the subsequent necrosis of body tissues.

Side effects include, bleeding, clotting (when given alone), teratogenic (fetotoxicity, NOT to be given to pregnant women).

The antidote for it is vitamin K, or we can just give fresh frozen plasma (FFP) because they contain already activated clotting factors.

P.S When we give warfarin we should keep an eye on the INR, the target is 2 - 3, if it's less than 2 then the patient is at risk for clotting (low dose), but if it's higher than 3 then the patient is at risk for bleeding (high dose) and in either way we should correct the dose, and also you should know that warfarin was first used as a pesticide for rats in 1948, and only in 1954 it was used on humans as anticoagulant.