For what it's worth, I'd say legalise it all. I don't smoke or drink myself, but I don't have a problem with anyone else doing whatever they want in the privacy of their own home. Does the fact that a lot of drugs are illegal stop people taking them? No. So why not make them legal, tax them to raise some money, and introduce the same kinds of rules that exist for alcohol and tobacco, where certain standards need to be met.
I agree with the point touched on by the previous poster, but would expand it. If someone commits ANY crime while under the influence of ANY kind of drug, I'd double the penalty for it, as well introducing an automatic death sentence or life-without-parole sentence (depending on where you are) for ANY drug related crimes that physically harm anyone else.