A commenter on one of Orac’s posts (& now I’m darned if I can remember which one) informed the others present that, while arsenic can be fatal for humans, it doesn’t kill rats. (It was part of a discussion on animal testing, which means the post was probably this one.) Now, I am a fan of the "Lord Peter Wimsey" books by Dorothy Sayers. One – Strong Poison – sees our hero attempting to clear a woman of using arsenic to murder her lover (a guilty verdict would have seen her sent to the gallows). It’s definitely not good stuff to consume in any quantity, although for a while arsenic in small amounts was regarded as a tonic in some circles.
But I also recalled that one source of the arsenic used for killing those whom the poisoner disliked, or wanted out of the way, was – rat poison. Another was fly-papers, which used to be hung to catch flies back when flyspray hadn’t been thought of. My recollection was confirmed when I came across this interesting post by Deborah Blum at Speakeasy Science, which discusses some notorious cases with arsenic as the murder weapon. (Deborah also muses on a question by another writer – why don’t we remember infamous personalities like Mary Ann Cotton, who killed at least 20 people, including some of her own children? She suggests, & I agree, that it’s a matter of choice "not to dwell [too long] in the darkest corners of human behaviour, to spend too much time in the company of aberrant personalities."
Arsenic remained a common choice for poisoners until the advent of the Marsh test, at which point it became rather straightforward to detect the presence of arsenic in the deceased’s remains.
As for the rats – it takes around 763 mg/kg (or 0.763 g/kg) of arsenic to kill your average rat, although this particular poison is not part of the modern rat-killers armamentarium.