[int_19h]

I should have made it more clear.

What I was talking about is the sale of substances that are 1) advertised specifically for consumption, and 2) have known highly harmful effects when consumed as advertised. This is similar to regulating poisonous food. You can still buy poison and food separately, and combine them if you really want to. So this is not about possession.

For possession, I can think of only two reasons why it should ever be regulated. One is when the substance requires special handling to store safely, and can e.g. leak into the environment and affect others if handled improperly. I'm not sure if there are any drugs that are in this category. Presumably, at least some of them could be in sufficient quantities. But even then, this would be regulating safe storage, not possession per se.

The other reason is when a substance, when consumed, is likely to make the person highly dangerous to others - a hypothetical example would be a drug that reliably induces violent rage, and that has no other legitimate use (e.g. not a beneficial drug in smaller doses).