Most rodent poisons are coumarin based. They kill the animals by inhibiting formation of blood clotting factors so they prevent the blood from clotting. The animals essentially bleed to death. This process can take several days to start working after ingestion since the already present clotting factors have to be used up. Since the rats/mice start becoming dehydrated through blood loss they get thirsty. Proponents of poison use claim that this thirst will drive the rodents to leave the home to seek water. Anyone who has used this stuff knows that most of the rats/mice still die somewhere inside (usually where you can't reach them) and stink very badly. For more info see
How Does Rat Poison Work? By Jerry Hall Leo At Isnare.com Ezine Articles