Two trials here in Charleston focused on treating combat veterans, sexual assault victims, and police and firefighters with PTSD who had not responded to traditional prescription drugs or psychotherapy. Patients had, on average, struggled with symptoms for 17 years.

After three doses of MDMA administered under a psychiatrist’s guidance, the patients reported a 56 percent decrease of severity of symptoms on average, one study found. By the end of the study, two-thirds no longer met the criteria for having PTSD. Follow-up examinations found that improvements lasted more than a year after therapy.

“We can sometimes see this kind of remarkable improvement in traditional psychotherapy, but it can take years, if it happens at all,” said Dr. Michael C. Mithoefer, the psychiatrist who conducted the trials here. “We think it works as a catalyst that speeds the natural healing process.”

The researchers are so optimistic that they have applied for so-called breakthrough therapy status with the Food and Drug Administration, which would speed the approval process. If approved, the drug could be available by 2021.

Under the researchers’ proposal for approval, the drug would be used a limited number of times in the presence of trained psychotherapists as part of a broader course of therapy. But even in those controlled circumstances, some scientists worry that approval as a therapy could encourage more illegal recreational use.

“It sends the message that this drug will help you solve your problems, when often it just creates problems,” said Andrew Parrott, a psychologist at Swansea University in Wales who has studied the brains of chronic Ecstasy users. “This is a messy drug we know can do damage.”