I can't even remember which story I read this in but I'm sure wolf shifters can be poisoned by silver as it is deadly to them. Obviously in small doses it won't kill but will cause serious health problems - such as Caleb has been suffering from!!!!
So, there I've solved your poison question!
But, the next question is when was Caleb poisoned (Montana or here)? AND is it still happening? If the latter, then who is doing this to Caleb???????
I see we're already on page 8 of comments and this is answering @kbois post from page 1!!
I've yet to read the subsequent pages so no doubt other posters may have reached the same conclusions as me.