Nigel McCourry removed his shoes and settled back on the daybed in the office of Dr. Michael Mithoefer, a psychiatrist in Charleston, S.C.

“I hadn’t been really anxious about this at all, but I think this morning it started to make me a little bit anxious,” Mr. McCourry said as Annie Mithoefer, a registered nurse and Dr. Mithoefer’s colleague and spouse, wrapped a blood pressure cuff around his arm. “Just kind of wondering what I’m getting into.”

Mr. McCourry, a former U.S. Marine, had been crippled by post-traumatic stress disorder ever since returning from Iraq in 2004. He could not sleep, pushed away friends and family and developed a drinking problem. The numbness he felt was broken only by bouts of rage and paranoia. He was contemplating suicide when his sister heard about a novel clinical trial using the psychedelic drug MDMA, paired with therapy, to treat PTSD. Desperate, he enrolled in 2012. “I was willing to do anything,” he recalled recently.

PTSD is a major public health problem worldwide and is particularly associated with war. In the United States, an estimated 13 percent of combat veterans and up to 20 to 25 percent of those deployed to Iraq and Afghanistan are diagnosed with PTSD at some point in their lives, compared with seven percent of the general population.

Although PTSD became an official diagnosis in 1980, doctors still have not found a surefire cure. “Some treatments are not helpful to some veterans and soldiers at all,” said Dr. Stephen Xenakis, a psychiatrist and retired U.S. Army brigadier general. As many as half of veterans who seek help do not experience a meaningful decline in symptoms, and two-thirds retain their diagnosis after treatment.But there is growing evidence that MDMA — the illegal drug known as Ecstasy or Molly — can significantly lessen or even eliminate symptoms of PTSD when the treatment is paired with talk therapy.

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