Three years later Daniel Kreitman still chokes up when he talks about what he saw, and how it changed him. Kreitman, an upholsterer by trade, had taken psilocybin, a hallucinogen derived from mushrooms, in a trial at Johns Hopkins University School of Medicine for nicotine addiction. He was 52, and he’d smoked between one and two packs a day for nearly 40 years. After his first psilocybin session, his urge to smoke was gone. During his third and final session, he had the vision that helped him quit for good.