JOHN HALPERN clearly remembers what made him change his mind about psychedelic drugs. It was the early 1990s and the young medical student at a hospital in Brooklyn, New York, was getting frustrated that he could not do more to help the alcoholics and addicts in his care. He sounded off to an older psychiatrist, who mentioned that LSD and related drugs had once been considered promising treatments for addiction. “I was so fascinated that I did all this research,” Halpern recalls. “I was reading all these papers from the 60s and going, whoa, wait a minute! How come nobody’s talking about this?”

More than a decade later, Halpern is now an associate director of substance abuse research at Harvard University’s McLean Hospital and is at the forefront of a revival of research into psychedelic medicine. He recently received approval from the US Food and Drug Administration (FDA) to give late-stage cancer patients the psychedelic drug MDMA, also known as ecstasy. He is also laying the groundwork for testing LSD as a treatment for dreaded super-migraines known as cluster headaches.

And Halpern is not alone. Clinical trials of psychedelic drugs are planned or under way at numerous centres around the world for conditions ranging from anxiety to alcoholism. It may not be long before doctors are legally prescribing hallucinogens for the first time in decades. “There are medicines here that have been overlooked, that are fundamentally valuable,” says Halpern.

These developments are a remarkable turnaround. Scientists first became interested in psychedelic drugs – also called hallucinogens because of their profound effect on perception – after Albert Hofmann, a chemist working …