The problem with it being poison is that if that's all it was it wouldn't even be able to pierce a Doll's skin. All weapons, or anything strictly from the human world, do nothing to Dolls, Marionettes, and Puppeteers. Except for Doll Hunter nailguns.

What I mean by that is that swords will bend, or even break, and bullets will just bounce off of them. Similar things happen to any other weapons. Dolls are totally invulnerable to everything but other Dolls, Puppeteers and Marionettes, and Doll Hunter nailguns.

So there really does need to be something special there, and the ingredient explanation seems like a logical one. Hmm.