Doctors have long hoped to discover a "morning-after pill" to blunt the often disabling emotional fallout from traumatic experiences. It appears they have had one on hand all along: morphine

Doctors have long hoped to discover a “morning-after pill” to blunt the often disabling emotional fallout from traumatic experiences. It appears they have had one on hand all along: morphine.

In a large study of combat casualties in Iraq, Navy researchers reported Wednesday that prompt treatment with morphine cut in half the chances that troops would develop symptoms of post-traumatic stress disorder (PTSD). Other opiates are likely to have similar effects, experts said.

In previous work, researchers had found that larger doses of morphine given to children with severe burns also reduced post-traumatic symptoms, such as flashbacks, depression and jumpiness. These symptoms have become lasting in about one in eight service members returning from Iraq.

About 53,000 troops returning from Iraq and Afghanistan have been treated for PTSD, a disorder in which someone who has endured a traumatic event keeps re-experiencing it and the fear it caused. Patients often have trouble with work, relationships, substance abuse and physical ailments.

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The new study, in The New England Journal of Medicine, supports the standard practice in settings such as the battlefield and emergency rooms, where morphine is often used readily. But experts say it may have implications for the timing of treatment and for a wider variety of traumas, such as those resulting from rape or muggings.

“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.

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.

Of the 696, 493 — about 70 percent — were given morphine, most within an hour of injury. Two years later, 147 had developed PTSD. Of the 203 not given morphine early on, 96 developed PTSD.

That worked out to a 53 percent lower risk of developing PTSD for those treated early with morphine. No other factor, such as the nature or severity of injuries, age or gender, had much effect on the chances of developing PTSD, said Troy Lisa Holbrook, the study’s lead author.

“This is just one paper, one analysis, but it’s exciting because of the strength of the finding,” Holbrook said.

The researchers and outside experts agreed the effect would have to be proved virtually beyond doubt before morphine was routinely given to prevent the mental disorder.

“I would be very reluctant to suggest any change in clinical practice,” said Holbrook. “We need to understand a great deal more how this appears to work.”

Her co-authors were Michael Galarneau, Judy Dye, Kimberly Quinn and Amber Dougherty.

Doctors have known of opiates’ effects going back at least to Homer, who in “The Odyssey” described a drug that “had the power of robbing grief and anger of their sting and banishing all painful memories.”

The drugs appear to blunt the emotional charge of traumatic memories in several ways. Most obviously, they kill the pain when it is most excruciating; often, they scramble the ability to recall what happened. Opiates also inhibit the production of a chemical messenger called norepinephrine, which is thought to enhance fear signals in the brain.

Material from The Washington Post and The Associated Press is included in this report.