Warfarin is the most commonly prescribed blood thinner and is used to prevent clotting and resulting embolisms.  Ironically, it was first developed as a rodent poison (causing massive internal bleeding when it is ingested), but since the 1950s has been used as a pharmaceutical (ironically, after someone attempting to consume rat poison in a suicide attempt survived it). It was first developed after animals died of internal bleeding after consuming mouldy hay, but the active ingredient was not identified until several years later.   

Warfarin is slower acting than heparin, but can be given orally (heparin must be injected) and last longer in the body leading to longer periods of time between doses.

Despite its effectiveness and widespread use, it does require precautions.  It interacts with many other drugs and even some foods.  For example, it interacts with many common antibiotics and alcohol.  Foods high in Vitamin K (such as leafy vegetables) also inhibit its action.  The patient's international normalized ratio must be constantly monitored to ensure the patient is receiving an appropriate dose.  It is contra-indicated for use during pregnancy as it can cause abnormalities in the fetus in the first three months of development.

Warfarin works by preventing chemical reduction of Vitamin K

See also Warfarin induced necrosis

Warfarin at Wikipedia

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