“This idea that medicine can be used in the wake of a trauma to diminish the risk of developing a significant psychiatric disorder is incredibly important,” said an expert who was not connected with the study, Dr. Glenn Saxe, a psychiatrist at Harvard Medical School and director of the Center for Refugee Trauma and Resilience at Children’s Hospital Boston who conducted the studies on burn patients. “If the findings hold up,” he said, “the implications are huge and go well beyond the military”  for example, to civilian hospitals, where victims of rape and other terrifying ordeals may benefit.

Dr. Saxe and other experts cautioned that any benefit must be stacked up against the drugs’ risks: they are habit-forming with repeated use, and can blur memories of events that can be life-changing.

In the new study, researchers at the Naval Health Research Center in San Diego reviewed detailed medical records of 696 troops who had been wounded in Iraq between 2004 and 2006, determining whether and when morphine was used in treatment. Military doctors used the drug for most serious injuries  generally in the first two hours after the injury  but sometimes administered others, like anti-anxiety medications.

The study found that 243 of the servicemen and women were given a diagnosis of post-traumatic stress within two years of their injury. When the severity of the wounds was taken into account, researchers calculated that the diagnosis was half as common in those who had received morphine as in those who had not.

Age, sex and the cause of injury did not significantly alter the findings, said Troy Lisa Holbrook, the study’s lead author.