Ketamine. It can cause hallucinations in higher doses (anaesthetic), but is used as an analgesic in lower doses. It is also used commonly in children to induce anaesthesia. I am using it more in the patients I am looking after in intensive care.
My Delphi had it in a drip post op before coming home and she was a bright as a button following what I thought was quite major surgery and it certainly did affect her appetite in a good way.