There should be a difference, however, between users of illegal drugs purchased for mind altering alone, and prescription drugs, when acquired without prescription for the purpose of pain relief.
In Rush's case it is quite provable that his addiction started by having these drugs prescribed for pain. The reason that buyers are seldom dealt with has to do as much with resources as anything else, it simply becomes more practical to target dealers, and dealers alone, when law enforcement attempts to deal with the drug problem.
This'd be a built-in defense for anyone busted for possession of illegal drugs. The defense attorney would conjure up some medical condition--discovered after an exam by a doctor hired by the defense attorney--and poof! The defense attorney says, "your honor, my client wasn't using drugs for recreation. My client had a previously undiagnosed medical condition and he was self-medicating. We move to dismiss the charges."
Sorry, I must respectfully totally and completely disagree. Your analogy is completely illogical. For example, "There should be a difference, however, between armed robbery committed for obtaining money, and armed robbery, when committed for the purpose of a thrill.". The motive is irrevilant; the crime was committed with full knowledge that the actions were outside the bounds of the law.
Millions of people use pain killers under medical care, and then pursue non-narcotic pain killers for pain managment. My wife went though Pancreatitus (rated the #1 most painful event a human can endure). She spent 10 days on Morphine, then went to codine; then asprin. She does not wear a cape and tights (well, except for Friday nites .... but I digress). I was on morphine during my Gal Bladder surgery. We both managed without becomming addicted. Rush CHOSE to pursue these illegal drugs, with the excuse that they were for pain management. If pain management were indeed the issue, a qualified MD would have kept Rush under supervision.