NEW YORK (Reuters Health) - A study of British troops shows that prolonged periods of deployment, above the recommended limit, harm mental health and lead to problems at home, particularly for troops on the front lines.

Among a random sample of 5,547 active duty men and women in Britain's armed forces, those who were deployed for 13 months or longer over a 3-year period, in breach of guidelines, were much more likely to show signs of post-traumatic stress disorder (PTSD) and to have problems at home during and after deployment, compared with troops deployed for shorter periods.

Soldiers made to stay on the front lines for longer than expected were also more apt to suffer PTSD, a finding consistent with a recent survey of US troops in Iraq, which found that soldiers who did not known when they were scheduled to return home had increased levels of psychological distress.

Longer than recommended deployments were also associated with severe drinking problems.

Direct combat exposure strengthened these relationships.

The relation between the number of deployments and other mental health disorders was less clear - and there was no link between number of deployments and marital problems.

British commanders have "raised concerns" about the ability of the armed forces to cope with simultaneous major operations in Iraq and Afganistan, the researchers note in the British Medical Journal Online First, posted on bmj.com.

The UK armed forces have recommended deployment levels called "harmony guidelines," which are intended to balance the need for rest and recuperation with deployment. Roberto J. Rona, professor of public health at King's College London, and colleagues suggest that deployments above the guidelines (13 months or longer over a period of 3 years) clearly have an adverse impact on psychological health.