Around one in 12 US military personnel exposed to combat in Iraq or Afghanistan have suffered from psychological trauma, a rate four times that of counterparts who were not sent to war, a study says.
"The unpredictability and intensity of urban combat, constant risk of roadside bombs, multiple and prolonged tours, and complex problems of differentiating enemies from allies can leave many troops with high stress levels and lasting health consequences," the authors warn.
Post-traumatic stress disorder (PTSD) is the term for a range of symptoms that include nightmares, memory flashbacks and chronic anxiety, with the individual often turning to alcohol or drugs for solace.
The paper, published online on Wednesday by the British Medical Journal (BMJ), is exceptional, in that it quizzed tens of thousands of military personnel about their psychological health before their deployment, and then asked them again, on their return.
Research into PTSD is typically done retroactively, which means it can be impossible to know whether an individual had some of the symptoms of PTSD, or was prone to them, before being deployed.
The authors, led Tyler Smith, director of the Pentagon's Center for Deployment Health Research, questioned 77,047 active members of the US military between July 2001 and June 2003 to launch a 21-year health study.
The volunteers came from all four military branches -- the Army, Air Force, Navy and Marines. They were asked to fill in a questionnaire about their psychological and emotional states and substance use.
Forty percent of the cohort were sent to Iraq or Afghanistan by 2006.
Between June 2004 and February 2006, the researchers carried out their first follow-up investigation. A total of 50,184 of the initial recruits responded.
The study found that PTSD developed among 4.3 percent of personnel who were sent to Iraq or Afghanistan.
But the rate was more than double -- 8.7 percent by one yardstick -- among those who had been exposed to combat.
Among those who had been deployed to Iraq and Afghanistan but not seen combat, the incidence of PTSD was 2.1 percent. It was 3.0 percent among those who had not deployed to either theatre.
In addition, PTSD was likelier among members of the army and among those who were female, younger, high-school educated or less, never married, divorced or black.
Higher rates were also found among those who reported at the start of the study that they were a current smoker or a problem drinker.
The authors say that the overall tally of US military with PTSD is not particularly high, but is concentrated in specific categories of personnel, which means that identifying those people at risk is essential for prevention and treatment.
Previous research has found symptoms of PTSD in as many as 30 percent of US Vietnam War veterans and in more than 10 percent of US military personnel returning from the 1991 Gulf War.
Often, these problems do not manifest themselves until several years after the end of the conflict.
"Concern is growing among the public and veteranS that post-deployment health consequences among US military personnel may be considerable and lasting," says the new study.
"Combat duty in Iraq has been associated with high use of mental health services and attrition from military service and possible alterations in neural functioning after deployment."