Coumadin (warfarin) is the active ingredient in rat poison
. That being said, Coumadin is dosed, for humans, in a manner which prevents blood clotting (especially for those at risk), and does not act as a poison at the therapeutic dosage level. (However, when rats ingest the high concentrations of warfarin in rat poison, they bleed to death, internally.)"
A longer, better and more precise answer is found on Wikipedia.