Rat poison is warfarin (also known as coumadin) which is a blood thinner. It works by not weakening rats per se, but by thinning their blood so that when they're squeezing through tight spaces, blood vessels in the skin break and rats bleed to death. Matter of fact, that seems to be the cause of the major complaint I've heard about warfarin: rats end up dying in people's walls and stink!

Warfarin is also taken by humans to help prevent blood clots. The medication needs to be taken every day in order to maintain a therapeutic effect, i.e. it has a rather short half life. I'd be the first one to caution against secondary poisoning, but I'm not sure rat poison is an instance where other animals are at risk.

I'm very happy to hear from Jamie that Barred Owls are feasting on rats!

Mike Donahue
Seattle
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