Talking to people on the internet from the USA it seems you go to a Dr and tell him/her what you want. If they say no you just move on until you find a Dr prepared to give you what you want.
As was pointed out at the trial - their relationship wasn't a typical doctor-patient one, but one where MJ demanded, and Murray duly provided with little thought for the consequences. Wasn't a surprise verdict to be honest, the evidence was stacked against him.