This is according to Science:

In people with PTSD, a small sensory trigger such as a sound or a smell can bring a traumatic memory rushing back. “The disabling element of PTSD is the fact that when the memory starts, the emotions completely override you and overwhelm the brain,” Nutt says. Studies suggest that MDMA can dampen the emotional response to the memory, allowing people to relive their trauma and work through it, he says. The MDMA-treatment consists of several sessions of psychotherapy, some conducted while the patient is under the influence of the drug…

A small U.S. study that first suggested MDMA could help treat PTSD was published in 2011. Since then, researchers in Canada, Israel, and the United States have jointly carried out larger phase II trials funded by MAPS; their results, which remain unpublished but have been reviewed by the FDA, were very good, says Doblin. Overall, 107 participants who had suffered from PTSD for an average of 17.8 years were treated in the phase II trials, Doblin says. Of the 90 patients who were available to be studied 12 months later, 61 no longer had PTSD.

In late July, says Doblin, MAPS and FDA agreed on how the coming phase III trials—usually the last hurdle before seeking a drug’s approval from regulators—should be conducted.