I have had many operations that have required me to take a pain med. Darvocet(or I guess darvon) was a great drug that didn't mess with my mind or cause me any sort of problem at all. It just relieved pain.
Once they removed that and moved to oxy I could not handle it at all. I had hallucinations with it and it left me in a drugged state.
I feel they did that so they could knowingly push a more addicting drug.