It’s not metaphorically rat poison - it’s literally rat poison. And rat poison with a sort of evil genius all its own. The idea is that rats eat it, the bleed internally, they get thirsty and they go outside in search of water which means they don’t die inside your walls etc.
Having said all that - it’s saved many, many human lives.
It’s not all that uncommon for something to be helpful in low doses and harmful or lethal in larger doses - happens all the time. Digoxin or Digitalis is one example that comes to mind. Tylenol is definitely another. Something as simple as potassium could be another.
“bleed internally”
She and Bill are worth about $150 million.
She can afford to get her clotting tested periodically.