I don't think the issue is so much that he used one of these drugs once, it's that he used each of them once, and initially on a schedule so compressed that it left nearly no time to purge and recover.

And while many of these drugs may be available with a prescription, I'd be surprised if any doctor worth his oath would prescribe this kind of sequence of drugs. (To say nothing of the fact that lasting side-effects with prescription drugs is more common than it ought to be.)