It's not toxic to poultry. It is slow to absorb and not that soluable in water, for injection only. I believe it's a different kind of "caine" from what I've read. I used it on a rooster a couple of years ago without any ill effects.
Procaine penicillin was the reason for my question. Just trying to educate myself and I keep finding conflicting information about its use in birds. Found a couple of places that said it's toxic, but lots of people here use it.