What medication was once commonly used as rat poison?

Coumadin is the brand name for warfarin, an anticoagulant originally used as a rodenticide. It inhibits vitamin K epoxide reductase, blocking the activation of clotting factors II, VII, IX, and X. Rats that consume it repeatedly die from internal bleeding—exactly why it was effective as rat poison. The drug’s story began when cattle bled to death after eating moldy sweet clover; scientists isolated dicoumarol, then developed warfarin (named for the Wisconsin Alumni Research Foundation, WARF). Later, it was adapted as a human “blood thinner,” used with careful INR monitoring. By contrast, Aspirin is an antiplatelet pain reliever, Tylenol (acetaminophen) relieves pain/fever without affecting clotting, and Eliquis is a modern anticoagulant never used as rodenticide. Memory tip: WARfarin = “war on rats.”