Why should personal consumption, or possession with intent to consume personally, ever be criminalized? No matter how harmful the substance is to the person, the basic principle of bodily autonomy (you know, the same one that dictates abortions to be legal) applies here.

The only reasonable exception I can see is substances that alter behavior in a way that's likely to endanger other people (e.g. those that induce aggression). Ironically, though, alcohol would be relatively high on that list compared to most everything else.