Ketamine is legal for doctors to use on humans in most parts of the world, but most humans object to brain biopsies while alive, or injections directly into the brain while the instruments are monitoring what happens in the skull.

That's one reason that the method of action for plenty of approved treatments for mental health can be "???" - human clinical trials mostly check that it's not going to {kill you, make it worse} and works better than placebo, not "what's going on inside this live human brain", until we get better live monitoring non-invasive tooling for this sort of thing in humans.