The drug floods the brain with hormones and neurotransmitters that evoke feelings of trust and well-being, users report. Researchers say this allows patients to re-examine traumatic memories.

In follow-up psychotherapy, patients process emotions and insights brought up during the MDMA session. The current protocol calls for patients to take MDMA two or three times, each a month apart, interspersed with psychotherapy.

“The MDMA alone or the therapy alone don’t appear to be as effective,” Dr. Mithoefer said. “The MDMA seems to act as a catalyst that allows the healing to happen.”

What do patients say about it?

“I was actually able to forgive myself,” said Nigel McCourry, 36 a Marine veteran who was deployed in 2004 to Falluja, Iraq, whose experiences mirrored those of three other patients interviewed.

Mr. McCourry came home from war unable to escape scenes of an explosion that nearly killed him, and haunted by the memory of two young girls he accidentally killed in a fire fight. He struggled to sleep. He drank to forget. Rage eroded most of his relationships.

He tried help at a Veterans Affairs hospital, but couldn’t let his guard down enough to benefit from standard psychotherapy. A handful of medications meant to help left him feeling like a zombie, and he gave them up. He was contemplating suicide when he tried MDMA.

“When it kicked in, it was like an epiphany,” he said. “I could see all these things from combat I was afraid to look at before, and I had a totally new perspective. I relived the parts of me I had lost. I realized I had viewed myself as a monster, and I was able to start to have some compassion for myself. It was a turning point, and for the next year I continued to get better.”