I am not sure about the new poisons, but Rat poison used to be just a mega-dose of Warfarin. It is an anti-clotting drug that causes animals and humans to have a hemorrhagic stroke if too much is ingested. It is dangerous stuff and that is why, if it is prescribed to a human (which it still is), it must be routinely monitored to ensure that the clotting factor (the INR) is maintained within an acceptable range. It is rarely just a "take a pill a day drug". You may be told to take one pill one day, 1/2 pill the next, and no pill at all on the third. The newer classes of drugs are more forgiving but still have their risks.