I should have clarified, and have done so now, that I meant that research was stopped for a long time. Since the early '00s research has finally begun again:

https://www.apa.org/monitor/2010/11/psychedelics.aspx

http://www.theglobeandmail.com/technology/science/brain/high...

http://www.psychiatrictimes.com/articles/research-psychedeli...