The medical user starts off doing something that the doctor prescribes to help him. He is not doing it to get high, but to control pain. The intent is completely different, even if the result may be the same.
A good friend of ours failed at rehab the first two or three times because she thought because she was addicted to "prescription" drugs (opiates) that she wasn't as addicted as someone who did cocaine--she did get them "legally," from at least four different doctors in three different cities--in addition, of course, to stealing drugs from friend's medicine cabinets, stealing one doc's prescription pad and a whole lot more. When she finally realized that yes, indeed, she was as addicted as the person who did cocaine was she able to rehab successfully.