Okay so this post reminded me of a headcanon I had.
I would 100% believe that after entering Pomefiore, Rook would start microdosing bits of poison here and there to see if he could build up an immunity. (A practice called mithridatism, named after King Mithridates VI who was known for doing just that. There was apparently even a play written about him in the 1600s. I could absolutely see our resident unhinged theatre history buff, Rook, knowing about the Twisted Wonderland equivalent of this and deciding to try it for funsies.)
So, by the time we get to current day, something as potent as the cursed apple juice in Chapter 5 would absolutely do him in and it is still buck wild that he would try to drink it and not toss it… but also, he could probably down a bit of belladonna as a party trick.
Funniest thing about modern gravity falls fans is probably the fact they're seeing so much scientist yaoi recently and they'll come into the show completely spoiled. however, 2012 gravity falls fans did not have the luxury of knowing they would draw gay fanart of this thing