I am not sure if peanut butter contains canola oil or not, but canola oil can kill rats. It causes degeneration of the heart, kidney, adrenals and thyroid glands. Just thought I'd mention that, in case there is canola oil in PB.

Peanut butter can also choke rats, since it is sticky, and since rats can't vomit, they could suffocate.

I have never heard of peanut butter causing bone issues, but I wouldn't be suprised if it did- seems like every kind of food is deadly to rats.