For more than 40 years, Charlie Bessant was that guy.

The guy who’d slink out of every party to catch a smoke on the sly. Who’d walk for blocks late at night through his dicey DC neighborhood to score a pack of Marlboro Lights, no matter the weather.

Bessant tried and failed to quit many times. He used the nicotine patch, gum—you name it.

“I always thought, ‘I’m going to quit—this is my last pack,’ ” says Bessant, 64, now living in Silver Spring and semiretired from his work making mounts for museum exhibits. “A big percentage of packs I bought were the last pack I ever bought.”

But on a fall day in 2009, Bessant walked into a building at Johns Hopkins Bayview Medical Center in Baltimore. In Room 3102, he was given a dark-blue capsule to swallow. He lay down on a couch and nestled inside a cocoon of blankets. He put on eyeshades and headphones and waited for the blue capsule to do its thing.

And he never smoked another cigarette again.

Bessant’s method of kicking his cigarette habit was unorthodox, to say the least: The blue capsule contained a healthy dose of psilocybin, the active ingredient in hallucinogenic—a.k.a. “magic”—mushrooms. Bessant was a volunteer in the world’s only study investigating whether psilocybin can help cigarette smokers quit. Although most people associate the drug with tie-dye-clad college students and Phish concerts, scientists are discovering that when used in a carefully controlled research setting, psilocybin may achieve something far more powerful than a fleeting, rapturous high. It may actually change lives.