Mental disorders are common in the UK military, especially alcohol problems and neurotic disorders. PTSD remains relatively uncommon. There is no health effect of deploying during the 2003 invasion of Iraq (TELIC 1) for regular personnel, but reservists who deployed on TELIC 1 and other recent non-TELIC deployments are at an increased risk of PTSD symptoms compared to reservists who do not deploy.
The high prevalence of alcohol problems is consistent with our previous reports . Depression is also common (as it is in general population studies and the US studies reported below) although major depressive disorder is less common than milder depressive disorders. Panic disorder is rare, presumably individuals who suffer from severe panic symptoms would have difficulty in completing routine operational duties, pre-deployment training, or pre-enlistment screening. Somatisation disorder was uncommon which is consistent with the lack of increased prevalence of medically unexplained symptoms associated with deployment to Iraq in contrast to the 1991 Gulf War . However, recent data shows that reporting of (all) symptoms has increased since the 1991 Gulf War (Horn O, Sloggett A, Ploubidis GB, Hull L, Hotopf M, Wessely S, Rona RJ: Upward trends in symptom reporting in the UK Armed Forces submitted 2008).
Associations of common mental health problems and PTSD symptoms
Young men, those with pre-enlistment vulnerability and those who have been separated, divorced or widowed were at increased risk of common mental disorders. It has already been well-documented that such groups are at greater risk of mental health problems within the military [25, 26].
For regular personnel, we did not find an overall health effect of deployment to the main war fighting phase of the Iraq War which contrasts with US findings [4, 6]. However, in common with our previous study , we found a higher prevalence of PTSD symptoms in reserve personnel who deployed on TELIC 1 or other recent non-TELIC deployments when compared to non-deployed reservists. We have proposed that the increase in mental health problems in Iraq deployed reserves may be due to a higher perceived exposure to traumatic experiences in theatre, lower unit cohesion and morale amongst reservists, more marital discord during deployment and greater difficulties adjusting to life on homecoming .
Comparison with the general population
Direct comparison with a non-military population is not possible as the PHQ has not yet been used in large scale epidemiological surveys in the UK. The most comprehensive survey of the mental health of the UK general population occurred in 2000 utilizing the Clinical Interview Schedule - Revised (CIS-R) . The prevalence of neurotic disorders (generalized anxiety, depression and panic) in the UK population is 16.4% compared to 13.0% in this military sample. We would expect the prevalence of neurotic disorders to be lower in the military because of the screening procedures prior to enlistment, and the discharge of the most unwell after recruitment. Prevalence estimates of depression were similar between the military (11.0%) and the general population (11.0%), as was panic disorder (military 1.1%, general population 0.7%), major depression (military 3.7%, general population 2.6%) and somatisation (military 1.8%, general population 2.6%).
Comparison with other military populations
1991 Gulf War Studies
After the 1991 Gulf War, a series of case-control studies, comparing the health of Gulf and non-Gulf deployed personnel were conducted [29–31], including detailed clinical psychiatric assessment . In spite of methodological differences, amongst non-disabled Gulf veterans, Ismail et al  reported that the four week prevalence of major depressive disorder was 3% (compared with 3.7% in our study), the prevalence of panic disorder was 1% in comparison to 1.1% in our cohort, and the prevalence of any anxiety disorder was 3% in contrast to 4.7% in our cohort. The major difference was in relation to alcohol problems. Ismail et al  report a prevalence of 7% for alcohol dependence and 3% for alcohol problems, in contrast to a combined prevalence of 18.3% in this cohort. The difference may be due to changes in the culture of drinking in the UK in general and the Armed Forces in the last decade [33, 34], although the measures used were different in the two studies. Prevalence of PTSD in this study is higher than those reported in 1995 but the increase in the UK has been modest. Finally, Ismail et al  reported prevalence of somatoform disorder of 18.0% in unwell Gulf veterans and 6.0% in well Gulf veterans in comparison to rates of 1.8% in this cohort. This is consistent with the lack of an increase in medically unexplained symptoms after the 2003 Iraq conflict, in contrast to the 1991 Gulf War , after which there was an unexplained increase of medically unexplained symptoms (Gulf War Syndrome).
Contemporary US Studies
Riddle et al have reported on the prevalence of common mental disorders in a large military cohort in the US (The Millennium cohort) . In spite of the methodological differences in sampling and some of the instruments used prevalence between the UK and the US cohorts are similar. Alcohol abuse was the most common diagnosis in the two studies (12.6% in the US versus 18.0% in the UK). The prevalence of major depressive disorder and panic disorder were similar (3.2% (US) versus 3.7% (UK) and 1.0% (US) versus 1.1% (UK)). The prevalence of other anxiety disorders was lower in the US when compared to the UK (2.0% and 3.8% respectively), whereas the prevalence of PTSD in our UK sample was 4.8% and 2.4% in the Millennium cohort.
We have previously reported the high prevalence of problem drinking in the UK military . It is possible that the differences in the prevalence of alcohol problems between the UK and US found here may reflect differences in the culture of drinking or differences in the rate of deployment in the two samples as alcohol misuse increases after deployment .
There were no significant differences in the prevalence of PTSD symptoms between the US and UK regular personnel within similar demographic and deployment groups in this study. US reserve forces reported more PTSD symptoms than their UK counterparts, but this difference became non-significant when combat experience was taken into account. It is unclear why UK reserves felt more at risk of being killed or injured than their US counterparts despite their lower combat exposure, but this may be explained by differences in training and experience between US and UK reserves.
Initial comparisons between US and UK prevalence of PTSD after the Iraq War revealed differences using an identical measure of PTSD . The current analysis supports Hoge and Castro's  suggestion that these differences are probably best explained by differences in demographics, military and combat experiences in the original study populations used for comparison.
Strengths and limitations
The strengths of this study are the relatively large sample and high response rate, with no evidence of bias in terms of health between responders and non-responders. The study used a structured diagnostic instrument, and did not rely on questionnaire self-report of symptoms or distress. However, this is a cross-sectional study thus causal relationships cannot be inferred.
Although our response rate was high, our sample was already based on a 61% response rate . It is possible that the lack of difference between responders and non-responders in both studies missed the most vulnerable, unwell or socially excluded members of the still serving/ex-military population.
For some of our subgroups, we had small numbers which inevitably has reduced the precision of our prevalence estimates. In contrast to other work, we reported lower rates of mental health disorder in female personnel. We are concerned that our results for women are likely to be distorted by the low numbers of women in the sample.
The comparisons that we make with US data are limited in several ways. First, although the data relate to the same Iraq deployment, they were collected in different ways. The PDHRA data was collected cross-sectionally in 2005-6 and enquiry was made about both exposure and PTSD symptoms at the same time. The UK data were collected at two time points (exposure was enquired about in the Phase 1 KCMHR military health study (2005-6)) and the PTSD data were collected 18 months-2 years later in the clinical interview study described in this paper. Second, the measure of PTSD used in the UK required endorsement of a Criterion A event in order to make the diagnosis, but this was not the case in the US study. Finally, our PTSD diagnoses were based on telephone interview rather than questionnaire report.
Although the PHQ is a well used measure, like all screens for mental disorders, it has limitations. Many argue that the existing measures in use for common disorders and PTSD are simply unable to sift out those with symptoms which result in functional impairment, and constitute disorders [38–40].
Although the study was independent of the military and results were entirely confidential, Service personnel may have been reticent to admit to mental health problems leading to an underestimation of true prevalence. This is particularly true of symptoms of 'alcohol abuse', the diagnosis of which may have disciplinary consequences for still serving personnel.