I think there's one other aspect of this issue that's not been addressed yet in this thread.

The role doctors play is not only ostensibly for the benefit of their advised patients, it's also to create a system whereby non-qualified people are explicitly barred from offering 'expert' advice on medical issues. While I can't disagree that nobody knows how your body reacts to substances as well as you do, I'm damn sure that non-doctors certainly don't know as well as doctors how well these substances will affect others. Were we to go to a non-regulated/minimally regulated marketplace for physio/psychotropic substances you would have legion such 'experts' dispensing cut rate advise.

There's a good deal of danger in the 'witch-doctor'/anecdotal advice approach to medicine. Is there possibly some up-side? Perhaps, but there are certainly some drastic downsides that outweigh any such upside (such as death from mixing incompatible medicines).

-Zeke