Then do you favor changing the law to codify this difference?
The doctor may be "guilty", but I don't see how the patient could be "prosecuted".
If the "patient"/addict is going through other channels to obtain prescription drugs then there are already laws for unlawful posession of prescription drugs.
There is still a difference in the criminal enterprises that exist to provide illegal prescription drugs and to manufacture/import/and distribute (plus control territory from competition) street drugs. Some prescription drugs are over the counter (or non-prescription medications) in other countries.
How much does the buyer's/user's actions set in motion a chain of events that causes great societal impact?