A new study into the herbal supplement St John's Wort suggests it is not effective as a treatment for depression.

Researchers from Duke University Medical Centre in North Carolina found it had no more impact on major depression than a placebo. Dr Jonathan Davidson, director of the Anxiety and Traumatic Stress Program at Duke, said: "Major depression is treatable. But this research suggests that major depression of at least moderate severity should not be treated with St John's Wort."

Herbal remedies have grown in popularity as consumers have become more interested in self-help, and more sceptical of conventional medicine, and the use of St John's wort has become widespread. This study is likely to cast widespread doubt on its efficacy.

Recent research also suggests that St John's wort may interfere with the ability of conventional drugs to work effectively. It may stop cancer drugs fighting tumours, and could even interfere with drugs that help the body accept transplanted organs.

Fellow Duke research Dr Robert Califf said that taking herbal remedies was fraught with danger, and stressed that just because a supplement was ineffective, did not mean it was harmless.

BBC News Online spoke to Professor Philip Cowen, a member of the British Association for Psychopharmacology, who said the study was in line with the prevailing opinion of most UK doctors, who generally believed St John's Wort to be an ineffective treatment for depression of any severity. However, Cowen added that it was unclear whether the herb may be beneficial to people suffering from milder depressions.

The Council for Responsible Nutrition, the trade association for manufacturers of dietary supplements in the US, dismissed the findings as "misdirected and inconsequential".

The study appears in the Journal of the American Medical Association. Read more about it by clicking here.

The website of the Council for Responsible Nutrition can be accessed here.