Woops got my codes confused. So the mentalitiser code is in the cave where the sewer entrance is. Check the weirdly placed bush in the middle of the room.... :D
I found it. I do not know why it never occurred to me to check the foliage in this game. Now I have a million more things that I need to check in case there are extra stuff. EDIT: I found Digest-Aid in an unmarked cabinet, and I continue my search.