Clinical trials have shown that St John's Wort is effective in treating mild to moderate depression, it was not found to be helpful for severe depression. It may also be helpful for anxiety, sleep problems and seasonal affective disorder (SAD).

It's not clear exactly how it works, but it seems likely that it affects the same chemical messengers in the brain as pharmaceutical antidepressants (ie serotonin and noradrenaline).

St John's Wort is available in a range of forms, but most people who take it as an antidepressant take it as a tablet and the recommended daily dose is about 900 mg of total extract. Like prescription antidepressants, it takes 2-4 weeks for the effect of St John's Wort to build up fully.

No treatment is free of side effects and this includes herbal remedies. The most common are an upset stomach, nausea and dizziness. St John's Wort is also known to interact with a wide range of prescription medicines including sleeping tablets and antidepressants. It may thin the blood and should not be used with drugs such as Warfarin and anticonvulsants; it may also interfere with the effectiveness of the contraceptive pill. It is therefore important to consult your doctor before taking it.