One reason rat poison works on rats, but less well on players, is rats can't vomit. Humans who ingest poisons often vomit them back up. Rats can't.
Web:
"Rats can't vomit. They can't burp either, and they don't experience heartburn. Rats can't vomit for several related reasons: Rats have a powerful barrier between the stomach and the esophagus. They don't have the esophageal muscle strength to overcome and open this barrier by force, which is necessary for vomiting."
HTH
