Usually it's the taker of bribes that is the biggest criminal. Any doctor who can be shown to have given addictive opioids to people that aren't terminally ill should be investigated.
This is completely regardless of whether it was the "standard practice" at the time, or whether some pharmaceutical sales rep told them it was a good idea.
If just a couple of hundred doctors would lose their license and a few dozen end up in jail, that would make some headlines and perhaps make other doctors think twice before prescribing oxycodone to someone with normal back pain. Doing that is like amputating someone at the hip for toenail fungus.