The four-year study will compare the herbal treatment with a placebo and depression drugs called selective serotonin reuptake inhibitors.

Researchers will assess changes in symptoms, functioning and quality of life in 300 participants over 12 weeks. Those who show no improvement will receive the active treatment they had not been assigned before, while patients with improved symptoms will continue treatment for an additional 14 weeks for a total of 26 weeks.

Costing more than $4 million, the multicentre study is being jointly funded by the National Institute of Mental Health (NIMH), the National Center for Complementary and Alternative Medicine (NCCAM) and the Office of Dietary Supplements.

Dr Matthew Rudorfer, associate director of treatment research at NIMH, said he hoped the trial would increase understanding of minor depression and offer "new evidence-based treatment recommendations for either primary care or mental health clinicians".

Minor depression is believed to be a significant health problem and a serious risk factor for major depression. However, the condition remains underdiagnosed and undertreated.

NCCAM Director Dr Stephen Straus said there was a great deal of public interest in herbal remedies for depression.

"This new study extends our earlier research efforts from the more serious form of depression to a clinically less serious one, yet one that is, in fact, of considerable public health significance," he said.

A spokeswoman for the UK charity Depression Alliance said the organisation welcomed the NIH study and hoped that successful trials in the UK would encourage doctors to offer more choice to people affected by depression.

"We believe that St John's wort can act as a powerful intervention, but as a herbal treatment it is readily available over the counter without medical advice.

"We are aware that there are companies who are striving to gain a license for their product in an effort to prove St John's wort's efficacy and safety and set the medical professionals minds' at ease," she added.

Minor depression affects approximately 7.5 per cent of the US population. Its symptoms are the same as those of major depression, though fewer in number and causing less impairment.

From Health-news.co.uk