Any sort of rational approach would start with legalizing use, and possession in amounts that are clearly intended for personal consumption, of any drug, excepting those that have immediate effects that are highly likely to be harmful to those in the vicinity (e.g. drugs that consistently induce violent hallucinations).

We can discuss transfer, sale and other forms of distribution on a case by case basis, depending on how harmful the drug in question is. And we can say that possession of significant quantities, clearly beyond personal consumption, should be considered evidence of intent to distribute (although it should really be rebuttable - i.e. the person should get a chance to present counter-evidence to prove that there was no intent to distribute, and the judge or jury should decide on the balance of probabilities).

But there's no rational reason whatsoever to punish consumption of anything.