“I think it’s a very important study” given how frequently the drugs have been prescribed, said Dr. Charles Hoge, a senior scientist at the Walter Reed Army Institute of Research, who was not involved in the study but wrote an editorial accompanying it. He added, “It definitely calls into question the use of antipsychotics in general for PTSD.”

The use of such drugs has grown sharply over the past decade, as thousands of returning soldiers and Marines have found that their post-traumatic stress symptoms do not respond to antidepressants, the only drugs backed by scientific evidence for the disorder. Doctors have turned to antipsychotics, which strongly affect mood, to augment treatment, based almost entirely on their experience with them and how they expect them to work.

To test those assumptions, a team of researchers affiliated with the Veterans Affairs medical system had 123 veterans with the disorder begin a regimen that added Risperdal to their treatment. Some of the patients served in Vietnam, others in Iraq or Afghanistan; all had tried courses of antidepressant treatment and found little relief.

After six months of treatment, these veterans were doing no better than a similar group of 124 veterans, who were given a placebo. About 5 percent in both groups recovered, and 10 percent to 20 percent reported at least some improvement, based on standardized measures.

“We didn’t find any suggestion that the drug treatment was having an overall benefit on their lives,” said Dr. John H. Krystal, the director of the clinical neurosciences division of the Department of Veterans Affairs’ National Center for PTSD and the lead author of the study.