• Medical interest in psychedelic drugs as treatments for illnesses such as anxiety, addiction and posttraumatic stress disorder has been renewed. • Small-scale studies involving human participants in the United States, Europe and Canada are showing that such research can be conducted in a safe and scientifically rigorous manner. • Preliminary findings show some successful results for these treatments, with significant clinical improvements and few — if any — serious adverse effects. • The emerging results may have implications for future medical and neuroscientific research, medical education and training, and public policy.