In the short term, psychedelics affect our serotonin receptors, the brain's main mood-regulating neurotransmitters. Next, they affect our glutamate receptors, which appear to produce the so-called transformative experiences and psychological insight that people experience on the drugs.

"This is the most rewarding work I've ever done," Bogenschutz said. "To see these kinds of experiences ... it's just not as easy to get there with psychotherapy."