Jonathan – An interesting and fair point, and I guess I can concede it in some respects. Certainly, pot (which I think should be legalized or at least de-criminalized) has become more potent due to prohibition. And it’s an interesting point about crack. Three factors, however, mitigate your point.

First, many illegal drugs are very impure basically because criminals sell them. Dealers “step” on coke and dilute heroin because they want to make as much money as possible by parting with as little of their commodity as possible, and honesty is not much of a consideration. In a legal market such lying would — presumably — be illegal just like any other false advertising or other efforts to mislead consumers. So, the products would be purer, which — you’re right — doesn’t necessarily mean more potent but it doesn’t necessarily mean less potent either.

Second, in a market where cocaine or PCP would be sold like cold medicine, competition and niche marketing would encourage some venders to sell “Extra Strength” heroin just as they sell Extra Strength Tylenol today (in fact, try to find regular strength Tylenol at an average store — it’s not so easy). Your points about the post-prohibition alcohol markets notwithstanding, we can still buy grain alcohol, whiskey (including God’s elexir, Jamesons) and other very potent spirits. And, I’m told, alcoholics drink these products regularly.

Which brings me to my last point. Sure, in a free and legal market there would be a broader range of lower potency drugs — call them zinfandel narcotics — which many of us could enjoy responsibly. But there’s something special about drug addicts: they’re addicts. And addicts often seek ever-greater highs and ever stronger drugs. Which brings me back to the begining. They are not rational actors.