Mancakes
Wiki says it was vinegar and the reaction was real, but the puke scripted.

jimmicampkin
Further to mancakes - all through rehearsals, the bottle had been filled with water but on the day of shooting Bruce Robinson filled it with vinegar and didn't tell him. His gasping reaction when he knocks it back is real, but the vomiting was in the script.

Auto-five for Withnail, one of the greatest films ever made.