At least at some point Rush let his physician know about his addiction because he made the statement that he is entering rehabilition after consulting with his doctor.
If his doctor was taking any sort of bloodwork from Rush, there were probably some readings that pointed to a continued use of Oxycontin over those 2 years.
We don't know if Rush was consuming more oxycontin than his doctor was prescribing or if his doctor had ceased prescribing oxycontin (or if his doctor was unwilling to continue writing prescriptions for oxycontin but told him that it would be helpful to remain on it if he could find alternate sources for the medication).
Certainly there have been doctors who have told patients that it may be helpful for them to get marijuana even though those doctors may have never written a "prescription" for it or noted that in the medical record.
Yeah, okay, agreed. But it would be interesting to know if he went to his doctor, or his doctor discovered his condition via lab tests/bloodwork, or his doctor read the papers and called him up asking "what the hell's THIS stuff I'm reading about?". :)
Maybe it's just me, but I can't envision a doctor saying "I can't prescribe this stuff for you any more even though you still need it: go find it on your own.". As long as there is supporting evidence (peer review, second opinions, corroborative diagnoses) that a medication IS need for a particular condition then a physician could prescribe a patient's continued medication until the end of time. They do that now for certain conditions.