... except cannabis, presuming you don't have a genetic disposition to schizophrenia. Nearly no one has ever died of a marijuana overdose. Compare that to alcohol (or, indeed, water). In fact, the only reason I say "nearly" is just in case; its safety has been known for a long time.
This is in fact one of the core reasons why I finally stopped being a straightedge and accepted taking it. (Though smoking is still a no-no and the worst way to experience anything, basically.)
There's a lot of things that are toxic at high levels but the typical volume avaliable commercially is never going to work, you need some kind of refined to the toxic elements only type of substance, which there's at least a possibility no one has figured out how to do that refinement, as there's no sense in it.