Transcendental meditation has been shown to decrease the severity of post-traumatic stress disorder (PTSD).

A trial involved 203 US veterans with PTSD, most with very severe symptoms, predominantly due to combat-related trauma.

After three months of treatment, meditation was found to be just as effective in reducing PTSD symptoms as the most commonly used psychotherapy, prolonged exposure therapy, and better than health education classes.

Current treatments for PTSD include cognitive behavioural therapy, group therapy, medication and trauma-based psychotherapy, or a combination.

Transcendental meditation - described as the effortless thinking of a mantra, without concentration or contemplation, to produce a settled, psychophysiological state of restful alertness - has previously shown promising results against anxiety and PTSD symptoms in military veterans and active personnel, but the studies have been uncontrolled or with small numbers.

The new study, which is published in The Lancet Psychiatry journal, is the first to compare transcendental meditation with prolonged exposure therapy.

Participants were randomly assigned to receive transcendental meditation (68 veterans), prolonged exposure therapy (68 veterans), or a control condition of PTSD health education classes (67 veterans).

Veterans with psychotic symptoms, mania, bipolar disorder, current suicidal or homicidal ideation, or cognitive impairment were not included.

Participants receiving medication for their PTSD could take part in the trial as long as their prescription had remained the same for at least two months prior to enrolment.

The transcendental meditation and health education classes were given in group sessions while prolonged exposure therapy was delivered one-to-one.

The severity of the veterans' PTSD symptoms were measured through an interview with the clinician at the start of the trial and after three months.

The study authors found transcendental meditation was just as effective as prolonged exposure therapy, showing similar reductions in PTSD symptoms, and both of the therapies were more effective than health education classes.

They found 68 per cent of participants were taking at least one PTSD-related medication, but in statistical analysis controlling for medication use, the authors found that transcendental meditation still reduced PTSD symptoms compared to health education, and was just as effective as prolonged exposure therapy.

Prolonged exposure psychotherapy focuses on re-experiencing the traumatic event through remembering and engaging with reminders of the trauma, as opposed to avoiding them.

While it is beneficial for many, 30-50 per cent of veterans participating do not show clinically significant improvements, and drop-out rates can be as high as 44 per cent.

New treatments, including options not involving exposure to the traumatic experience, are needed, and the study authors said their findings might offer a new non-trauma focused treatment for veterans, although they said further research was needed.