In many states, the possession of venomous snakes is illegal. Same goes for poison dart frogs.
I think the nuance is that opium and weed in small amounts won't kill you (and there is evidence to suggest that weed can't kill you at all, ever. I'm not entirely sure though).
Poision dart frogs on the other hand, you don't really get a "second try". Didn't realize they were illegal but I suppose it makes sense. Its poison after all.
Hemp is illegal in America for christ sake. Not because "its related to weed" either. Its because we can make anything out of hemp and that doesn't generate profit.
How anything naturally made by earth (weed, hemp, opium) can be labeled "illegal" is arrogant of the human race. What makes us think we know better than nature?
There are plenty of natural poisons out there that could kill us instantly. Why dont we make poison dart frogs illegal? Why aren't venomous snakes illegal?