Warfarin was initially introduced in 1948 as a pesticide against rats and mice, and is still used for this purpose today although better controls are available. The name Warfarin is derived from Wisconsin Alumni Research Foundation which funded studies, and coumarin. It is a synthetic form of an anti-coagulant originally discovered in spoiled sweet clover animal feed.
Patients being treated with Warfarin need to have their blood monitored on a regular basis, and since levels of vitamin K can affect the blood clotting ability, dietary intake of foods high in that vitamin (like leafy green vegetables) must be regulated as well.
This is yet another example of a "poison" being put to use in a therapeutic manner under tightly controlled circumstances.