Books by Mail and Poison

I never would have put these two items together--books and poison--until today.

One of our regular patrons (who I will say does have some mental handicap, but, just to be clear, this blog by no means is meant to make fun of or discriminate against said group of people) came up and looked a bit disgruntled.

Me: "Can I help you, sir?"
Him: "Yeah. I want to send my brother a book in your inter-library mail to him. Can you do that?"
Me: "Is this a book you have checked out?"
Him: "Yeah."
Me: "And you want to send it to his house?"
Him: "Yeah."
Me: "Ok. Well, the only books we send to patrons' houses are those that are bed-ridden or handicapped and cannot come to the library. And those people have to fill out a form, so if he would like to get books that way, he can fill out an application for that."
Him: "Ok. So I can't send it to him in your library mail?"
Me: "No sir. We don't really have a way to do that."
Him: (grumbles) "Ok. Well, what about if someone put illegal poison in my medicine? What can you do about that here?"
Me: "Umm...well, I don't know. That's probably something you want to talk to your pharmacist about."

Then he turns and leaves.

The thing about the whole conversation that left me befuddled was whether or not there was a kind of poison that was legal.