> If you mean Hallucinogens, they have been studied extensively and don't seem to be medically useful.
That's not true at all. "Hallucinogens" is a group of drugs so wide that it already includes many drugs already used in conventional medicine. Just go through the many drugs on this list[1] of hallucinogens, you'll quickly find references to potential medical benefits.
Just "potential benefits" though, since there hasn't been nearly enough research done on most of them yet. A lot of them haven't been scientifically studied at all, the basic work Shulgin did is the only things we know about them.
The most common hallucinogens (mushrooms, LSD, MDMA) were considered as very promising by many in the field until research of the potential benefits of them essentially was banned for decades due to the drug war, but that has begun to change now. [2][3][4]
Recent research indicate that certain hallucinogens may be very useful in treating a variety of medical issues - like depression, anxiety, alcoholism, cluster headaches, PTSD, etc etc. [5][6]
[1] http://en.wikipedia.org/wiki/Template:Hallucinogens
[2] https://www.apa.org/monitor/2010/11/psychedelics.aspx
[3] http://www.psychiatrictimes.com/articles/research-psychedeli...
[4] http://www.theglobeandmail.com/technology/science/brain/high...
[5] http://en.wikipedia.org/wiki/Psilocybin#Medical_research
[6] http://en.wikipedia.org/wiki/Lsd#Therapies