Mindfulness-based cognitive therapy (MBCT) may be just as effective as anti-depressants in helping prevent people with chronic depression from relapsing, scientists said on Tuesday.

Depression is one of the most common forms of mental illness, affecting more than 350 million people worldwide. It is ranked by the World Health Organization as the leading cause of disability globally.

Treatment usually involves either medication, some form of psychotherapy or a combination of both. Yet many patients fail to get better and suffer recurring bouts of illness.

MBCT was developed to help such people by teaching them skills to recognize and respond constructively to thoughts and feelings associated with relapse, aiming to prevent a downward spiral into depression.

In the first large study to compare MBCT and anti-depressants, researchers found little difference in outcomes.

In terms of cost, mindfulness training -- often viewed as more costly because it requires more time with a trained therapist -- was not significantly more pricey, particularly when given in group sessions, the study found.

Richard Byng, a professor at Britain's Plymouth University Peninsula Schools of Medicine and Dentistry, said that, while current standard treatment for chronic depression is to keep taking anti-depressants, many people don't want to take them for long periods and others want to avoid side-effects.

In this study, 424 adults with recurrent major depression who were on maintenance anti-depressant drugs were randomly assigned either to come off their anti-depressants slowly and receive MBCT or to stay on their medication.

While 212 patients continued taking their anti-depressants, the other 212 attended eight group mindfulness therapy sessions and were given daily home practice as well as an option to have four follow-up sessions over a 12-month period.

Study results published in The Lancet medical journal showed that after two years, relapse rates were similar in both groups -- at 44 percent in the therapy group versus 47 percent in the anti-depressant drug group.

"Whilst this study doesn't show that mindfulness-based cognitive therapy works any better than maintenance anti-depressant medication in reducing the rate of relapse ... these results suggest a new choice for the millions of people with recurrent depression on repeat prescriptions," said Willem Kuyken of Oxford University, who worked with Byng on the research.