If you have discussed everything...
Again, sneak past the warden who is reading, then save the game. Select the lithium in the inventory (makes your life easier), go back to the warden and quickly to the window and pour the lithium in the coffee. The warden will send you back. A cut scene will follow. After that, go past the warden (he is sleeping now) and enter the first door to the right. Search the warden.
Now go and find mitchells office. There you will find a bottle of bourbon, but also other things.