It is very important that people think twice about taking this drug

Dr Robert Califf

Researchers have conducted the largest ever clinical trial into the impact of the herb on major depression - a moderately severe form of the condition.

The researchers, from Duke University Medical Center in North Carolina, found it had no more impact than a dummy medicine.

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.

Danger

"Rather than self-medicate with an over-the-counter medication or supplement, patients are strongly advised to consult an appropriate healthcare provider to assess the best treatment for a depressive episode."

Fellow researcher Dr Robert Califf stressed that taking herbal remedies was potentially fraught with danger.

Symptoms of major depression

A depressed mood Loss of interest in normal activities that lasts most of the day nearly every day for at least two weeks Significant weight loss or gain Sleep disturbances, agitation or unusual slowness Fatigue or loss of energy Feelings of worthlessness or guilt Lack of concentration Recurrent thoughts of death or suicide

He said: "As long as these types of products remain available to the public without the protection of adequate, controlled and unbiased studies, taking them is like playing Russian roulette with your health."

Previous research have suggested that St John's wort may have a positive impact on depression.

But the Duke researchers said that these studies may not have clearly defined which types of depression responded well.

Other studies have shown that the herb can interact with other medications such as those to treat HIV, certain cardiac conditions and even those that keep the body from rejecting transplanted organs.

Cancer effect

Research published this week showed that St John's wort can also interfere with the ability of cancer drugs to fight and kill tumours.

The latest study involved 340 volunteers, who either given St John's wort, a recognised anti-depressant drug or a dummy medication.

Dr Califf said: "Just because St. John's wort was found to be ineffective for this type of depression does not mean it is harmless to the body.

"There are only two kinds of therapies - those that work and those that don't work.

Whether the herb might work in people with milder depressions many of whom do not present for medical treatment remains an open question

Professor Philip Cowen

"Until there is a reliable compound made from St John's wort that has demonstrated efficacy for treating depression, it is very important that people think twice about taking this drug."

Professor Philip Cowen, of Warneford Hospital, University of Oxford, and a member of the British Association for Psychopharmacology, said the study confirmed the prevailing view among UK doctors.

He told BBC News Online that doctors' common view was St John's wort was an ineffective treatment for depression of any severity.

Questions remain

But he said: "Whether the herb might work in people with milder depressions, many of whom do not present for medical treatment, remains an open question."

Professor Cowen said there were also doubts about whether the study had been particularly sensitive in detecting the antidepressant effects of treatment.

He said: "Perhaps the main message is that if people with depression are not helped by St John's Wort they should not give up on treatment but seek help from their doctor because other more effective therapies are available."

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

It accused the researchers of ignoring at least 30 other studies which have shown St John's Wort to be effective.

The study appears in the Journal of the American Medical Association.