The overwhelming majority of rat poison is a drug called warfarin. It's regularly prescribed by Doctors to patients who are at risk of developing a blood clot. The amount of warfarin someone would need to have the same effects as a rat are enormous and it's counteracted by sodium. Something most Americans eat too much of. Is the story possible, yeah, who knows how old it is and how many retellings it's been through. Would I take this as the gospel truth? No.