Karl Paul Link. Portrait of Karl Paul Link, American chemist and the inventor of warfarin, a chemical used as a blood anticoagulant in human medicine, & as rat poison. Link invented warfarin (3-(alpha-acetonylbenzyl)-4-hydroxycoumarin) in 1939 whilst working at Wisconsin University. This tasteless, colourless and practically odourless chemical kills rats by stopping their blood from clotting, so that they bleed to death after receiving minor wounds. Warfarin only needs to be consumed by rats in small doses to have its effect, reducing the risk of poisoning humans. The sodium derivative of warfarin is used in humans to treat abnormal blood clotting (thrombosis).