Good catch. Yes, he discovered it earlier when looking for morphine, but why did Tanya have it there? If you remember back when she dealt with the slow turner and how she originally got the bite mark, they had to put it down. Knowing she might have to do that more, she kept some supplies handy.

That being said, another details that I'm leaving out is what exactly they used. I know what it is, and I can leave people here to theorize, but I felt it was a sort of writer's morality to leave out the specifics.