After taking his shoes off, he returned to the office of the doctor in Charleston, S.C.

I hadn't been anxious about this at all, but I think this morning made me a little bit anxious.

After returning from Iraq, a former U.S. Marine named Mr. McCourry was crippled by post-traumatic stress disorder. He developed a drinking problem because he could not sleep. He felt that the numbness was broken by bouts of rage and paranoia. He contemplated suicide after his sister heard about a clinical trial using MDMA and therapy to treat post-traumatic stress disorder. He was desperate to enroll in 2012 and was willing to do anything.

Post traumatic stress disorder is a major public health problem and is 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 post traumatic stress disorder at some point in their lives, compared with seven percent of the general population.

Some treatments are not helpful to some veterans and soldiers, according to a retired U.S. Army brigadier. Half of veterans who seek help do not experience a meaningful decline in symptoms, and two-thirds retain their diagnosis after treatment.

There is growing evidence that the illegal drug MDMA, also known as ecstasy or Molly, can reduce or even eliminate symptoms of post traumatic stress disorder when used in conjunction with talk therapy.

The most encouraging results from the first of two Phase 3 trials were reported in Nature Medicine last year. The 90 participants in the study had suffered from severePTSD for an average of 14 years. Each received three therapy sessions with either MDMA or a placebo, which were overseen by a two-person therapist team. Sixty-seven percent of those who received MDMA no longer qualified for a post-traumatic stress disorder diagnosis two months after treatment, compared to 32 percent who received the placebo. In previous trials, MDMA caused no serious side effects.

The Multidisciplinary Association for Psychedelic Studies, or MAPS, sponsored the earlier Phase 2 trials of MDMA-assisted therapy, which were conducted between 2004 and 2017. Fifty-six percent of Phase 2 participants no longer meet the criteria for post-traumatic stress disorder after undergoing several therapeutic sessions with MDMA. After a year, the figure increased to 67 percent.

Mr. McCourry still counts himself among the successes a decade later. He had his first MDMA session in 2012 under the guidance of the Mithoefers, who have worked with MAPS to develop the treatment since 2000. He shared the video of that session with The New York Times.

The second Phase 3 trial is expected to be completed by October.

Rachel Yehuda, a professor of psychiatry and neuroscience at the Icahn School of Medicine, said that this approach represents real hope for long-term healing.

The chair of Psychiatry at the Yale School of Medicine said that the current moment is different from 20 years ago because of the widespread recognition that we should leave no stone unturned in identifying new treatments. The results so far from the second Phase 3 trial are very encouraging.

ImageDr. Michael Mithoefer and Annie Mithoefer have used MDMA doses in their therapy sessions. Though PSTD is a common diagnosis among veterans, there is no catch-all cure or treatment.
Dr. Michael Mithoefer and Annie Mithoefer have used MDMA doses in their therapy sessions. Though PSTD is a common diagnosis among veterans, there is no catch-all cure or treatment.Credit...Travis Dove for The New York Times

He is from a military family and lives in Portland, Ore. He joined the Marines because he wanted to make a positive difference, and he felt like he was there because of it.

But soon he became discouraged. He guarded oil convoys rather than fighting for freedom. He saw people being killed. He suspected that the explosion that knocked him unconscious may have caused a traumatic brain injury. The Army lacks tests that can objectively distinguish between traumatic brain injury and post-traumatic stress disorder, so he never received a diagnosis.

I put my life in danger and watched friends die for nothing.

The man was killed in a firefight two months into his deployment. He saw a white truck approaching from the opposite direction. The truck kept approaching despite being told to stop and firing a warning shot.

Mr. McCourry was shooting at it. The people in the truck were a father and his two daughters. The father survived, but the girls did not.

Between tours of duty, Mr. McCourry sought help from a battalion medical officer for his sleep and anxiety issues. When the doctor dismissed his concerns, Mr. McCourry lost his mind and started yelling at him. He was discharged on the basis of a personality disorder that was not legitimate grounds for discharge.

Mr. McCourry was overjoyed to be home, but soon realized that something wasn't quite right. He was tense with his friends and family. He found it difficult to control his anger because he was offended by any disrespect. He felt a mixture of numbness and guilt when he learned that nearly his entire squad had been killed by a roadside bomb.

Soldiers don't like to reveal that they have any problems, so they tend to minimize their symptoms after returning from war.

There is a phenomenon called moral injury, which can be difficult to treat, and is experienced by some veterans. According to Dr. Robert Koffman, moral injury develops in service members who feel responsible for perpetrating or for failing to prevent an act that violates their deeply held beliefs. The result is feelings of shame and guilt.

For a long time, vivid nightmares and paranoia prevented Mr. McCourry from sleeping. He went to a Veterans Affairs clinic. He was diagnosed with a severe case of post traumatic stress disorder, and the doctors recommended a number of treatments.

Paula Schnurr, executive director of the V.A.'s National Center for PTSD, said that these treatments bring relief for some patients, but they are not effective for all.

Around 40 percent of active duty military and veterans drop out of treatment for post traumatic stress disorder, according to some research.

He said therapy didn't help at all. The medications he was prescribed made his symptoms worse by causing serious side effects.

For those who do not find relief through available treatments, it can become chronic, disabling and even life threatening. 17 veterans die by suicide every day, according to Dr. Koffman.

I wanted the suffering to end, and there was no way to improve it. I wanted to die.

Dr. Michael Mithoefer, left, and Annie Mithoefer have been working with MAPS since 2000. Dr. Mithoefer likened MDMA-assisted therapy to immunotherapy for cancer: “We’re stimulating the body’s own capacity for defense and healing.”Credit...Travis Dove for The New York Times

When he heard about MDMA-assisted therapy, he was skeptical. He met with the Mithoefers for three 90-minute sessions to establish trust and give guidance on how to respond to difficult memories and feelings during treatment.

The sessions would last eight hours. The study's double-blind protocol did not allow Mr. McCourry to know what dose he would be assigned. The possibilities ranged from a low 30 to a high 125 gram dose. Mr. McCourry fell in the middle with 75 milligrams.

On the day of his appointment in 2012 he was reassured by the doctor that there was no agenda.

He said that it was just to have greater depth of understanding of mental processes and why he thinks the things he does.

He drank a swig of water and laid back after swallowing the pill. The room was filled with music.

After about an hour, a warm sensation began to wash over Mr. McCourry, and the music sounded more beautiful than before. Even though he was starting to worry about where things were going, he was able to relax.

The tone of the music was no longer inviting. He thought about asking the people to stop the music, but he remembered that he was supposed to breathe into it.

He felt a knot in his chest. He wondered why he had behaved the way he did when his friends tried to be kind to him. He was willing to explore those questions with the couple.

He removed the eye shades and described the new hardness he had developed after returning from Iraq.

"What if you just allow people to be nice to you?" she asked.

I would have to give up control of my life in some situations.

How would that look, giving up control? If someone is trying to be nice to you.

It could be a good experience, but I don't even consider it before I put up the walls.

According to Dr. Yehuda of Mount Sinai, trauma can cause lasting changes in genes, hormones and the brain. People with post-traumatic stress disorder often show exaggerated levels of stress hormones, and tend to have heightened activity in the amygdala, the brain region associated with processing threats and danger.

Negative experiences can change the body so much that it's possible that positive experiences could do the same. MDMA-assisted therapy seems to provide a transformational reset for many people.

Taking MDMA on its own does not mean that it will alleviate the symptoms of post-traumatic stress disorder. The drug seems to help a patient with their psychological healing.

The process stimulates the body's own capacity for defense and healing.

Scientists don't fully understand how MDMA works. Evidence in mice indicates that the drug opens a window in the brain that is better able to learn.

The senior author of the findings said that the critical period explanation really offers a different way of thinking about it.

Mr. McCourry realized that the reason he was shutting people out was because he had to trust them and give up control. In Iraq, self-reliance and distrust of others helped keep him alive. Those tools were now being criticized.

As the three of them talked through the revelations, Dr. Mithoefer said that it was whatPTSD is, really.

John Reissenweber saw combat in Vietnam but considered PTSD a weakness, until his wife, Stacy Turner, encouraged him to see a psychiatrist.Credit...Marissa Leshnov for The New York Times

Not everyone has the same experience with MDMA-assisted therapy.

John Reissenweber accidentally killed a 2-year old boy while serving in Vietnam. He came home and was always on edge and had one of the most acid tongues there were. He turned to alcohol for solace because he felt a constant need for control.

Mr. Reissenweber didn't think that post-traumatic stress disorder might have explained his behavior. His previous mind-set held that he was weak because of his post-traumatic stress disorder.

Mr. Reissenweber was diagnosed with post-traumatic stress disorder after his wife convinced him to see a doctor. His mental health did not improve despite regular appointments. He was in the Phase 3 MDMA-assisted therapy trial.

Mr. Reissenweber was worried that the drug would cause him to come undone, but in the weeks after the session, he felt more connected to himself. The second session went well.

He said that he could take a walk outside and feel the air against his skin.

Mr. Reissenweber decided to face his trauma in a black pit in the third and final session. He was terrified when he didn't pass through the pit into the light.

Mr. Reissenweber was not able to sleep for over a week. He realized that the pit represented his anger and hurt.

Mr. Reissenweber said his experience with MDMA-assisted therapy changed his life for the better. He calls his spouse his guardian angel because he finds traditional therapy to be productive.

He said it made him realize there was a reason for his hurt and his fears.

Mr. McCourry with his dog, Kiko, at home. He recently became a father and, after a 10-year struggle, had his discharge order changed to combat-related PTSD.Credit...Amanda Lucier for The New York Times

After his first session of MDMA-assisted therapy, Mr. McCourry emerged with a map of his mind.

He slept well that night, and his sleep problems never came back.

In one of his later sessions with MDMA, he realized that he had become so self-loathing for the person he had become in Iraq that he had accidentally killed two girls. He was able to replace the contempt he had forNigel the Marine with compassion.

After a long bureaucratic battle, Mr. McCourry was able to convince the Navy to correct his reason for discharge to combat-related post-traumatic stress disorder, instead of passive- aggressive personality disorder.

He said that sometimes he becomes overwhelmed in stress and starts to mentally shut down. He is able to better manage his feelings now that he knows when this is happening.

It is important for me that these experiences are used to show people that there is hope.