I think it's like one of two criteria: 1. Damaging to your body and highly addictive. This is most opioids, arguably tobacco (which is kind of grandfathered int

I think it's like one of two criteria: 1. Damaging to your body and highly addictive. This is most opioids, arguably tobacco (which is kind of grandfathered into being legal), meth, cocaine etc. 2. Can cause significant, unpredictable shifts in behavior in otherwise stable people, to the point of endangering themselves or others. This is (also) cocaine and meth, arguably alcohol (also grandfathered in), ecstasy, and why LSD is so highly scheduled. There's probably a world in which there're responsible, safe ways to experiment with LSD, but it's not something you necessarily want your given random guy on the street to be taking, lest he walk or drive into traffic or start swinging a 2x4 at people.