Bottom line: hard drugs fall into the same category as plutonium. Principles need to have exceptions made to them, where there would otherwise be tremendous harm to innocent people. Libertarians don't support people's right to buy/sell/use personal supplies of plutonium, and there's nothing wrong with treating hard drugs the same way.
The world is full of inherently dangerous substances & objects. We have decided, in the constitution, to give states the power to regulate [with due process] public use/possession of such property - and - to criminalize their misuse.
We cannot allow the state to have the absolute power to prohibit, as it is absolutely corrupting.
No, I said that prohibitions against easily produced substances which also cause relatively little harm in the grand scheme of things aren't practical or necessary.
Neither plutonium nor most hard drugs can be produced by their individual end-consumers, thus prohibiting them is a lot more practical. They are also MUCH more dangerous than things like marijuana or alcohol, thus justifying an exception to basic libertarian principles. A few hard drugs, such as meth, are pretty easy to produce and still extremely harmful, but the producers are not generally significant users (sort of self-regulating -- if one becomes a heavy user, one will shortly lose the capacity to produce). Most importantly, enforcement of laws against these more dangerous substances can be effective by focusing primarily on people who are producing and selling the substances for profit. With things like marijuana and alcohol, taking out the commercial producers and sellers simply results in most of the users taking up home production, therefore the prohibition accomplishes virtually nothing, unless we start to allow random searches of private homes.