By Clare Murphy
Health reporter, BBC News
Rates of PTSD have remained stable, the researchers said.
Alcohol misuse is a problem among UK troops who have been in Afghanistan and Iraq, but serious mental disorders are not as common as feared, a study says.
Post-Traumatic Stress Disorder (PTSD) rates are low despite many deployments in foreign combat zones, a study in The Lancet based on 10,000 personnel found.
They are significantly lower than those reported among US troops, it adds.
The King's College research, funded by the MoD, said more emphasis should be placed on drinking problems than PTSD.
The study, headed by Dr Nicola Fear and Professor Simon Wessely, surveyed nearly 10,000 regular personnel and reservists, asking them about their mental health and drinking habits.
It followed nearly a decade of the British being engaged abroad, including a six-year military mission in Iraq and ongoing operations in Afghanistan.
The prevalence of possible PTSD was just 4% - stable from the last major study carried out between 2003 and 2005.
It was slightly higher among the regulars who were deployed, but the high rates that some had feared as a result of ongoing missions and multiple deployments have not materialised, the study reported.
This is in sharp contrast to the US, where some estimates put rates of PTSD as high as 10-15% among those who have seen action.
While differences in the intensity of combat may account for some of this gap, the fact that US soldiers must show they have been harmed in service to access free medical care may inflate some of the figures, the researchers suggest.
At 20%, less severe mental disorders such as depression, anxiety and insomnia were not insignificant, but probably at rates lower than in the general population.
But there had been an impact of nearly a decade of combat operations, and this was most notable with the misuse of alcohol, the researchers said.
Robert Marsh, of the charity Combat Stress which helps veterans with PTSD, said the condition manifests itself in "a myriad of ways" including flash backs, nightmares, raised anxiety and depression.
He said: "We've seen a 66% increase in the number of new veterans seeking our help, and whilst the rate of people suffering from psychological injury in Iraq and Afghanistan remains low, across the piece - 180,000 individuals having served in those theatres - it's still quite a large number of people.
"On average veterans come to us 14 years after they have left the service and they come to us with a myriad of problems, not least of all self-medicating with alcohol and, or, drugs."
Overall 13% of those surveyed reported regularly drinking alcohol to excess, with those regulars who had been deployed more than 20% more likely to have problems.
Professor Wessely said the issue had to be treated with care as alcohol did play an important role in soldiers' lives.
"It plays a key role in bonding, as well as relieving anxiety and stress. But there is no such thing as a free lunch."
But he stressed that while rates for all problems were higher in those deployed than those who had not been, the number of deployments did not seem to affect the chances of reporting a mental disorder.
This may be explained by the "healthy warrior effect" - in other words, those who are the most psychologically resilient go on to further deployments, while those who are unwell do not.
And while rates of mental disorders were not rising, Professor Wessely warned that by the very virtue of the increasing numbers who have served in recent years, more will eventually require the help of the NHS - which will need to anticipate this increase.
An accompanying Lancet editorial noted: "Contrary to reports in the mainstream media, there is not an epidemic of mental health problems in this group of service personnel."
But it added: "This group is at increased risk of the social complications of excessive drinking such as violence and relationship breakdowns. Although having introduced some alcohol-control policies, the armed forces need to reassess whether they are rigorous enough."
The Ministry of Defence, which funded the independent research, said it took issues of problem drinking very seriously.
A spokesman said: "Individuals identified as being at risk from alcohol problems receive counselling and welfare support. This can include attendance on preventative early intervention programmes designed to alert them to the harm that alcohol can cause to themselves and others.
"More serious cases are treated through specialist medical and psychological treatment and rehabilitation, including where appropriate as in-patient. The consumption of alcohol is strictly regulated when personnel are serving on operations and disciplinary measures are taken where necessary."