Feeling blue? According to Dr Klaus Linde from the Centre for Complementary Medicine in Munich, a supplement of St John's Wort should improve your mood. In his review of 29 studies on the yellow-flowering hedgerow plant, also known as hypericum perforatum, Linde has shown it to be as effective as drugs such as Prozac in treating depression.

Around two million Britons take St John's Wort regularly and it has been widely studied. Possessing a number of active ingredients, including hypericin and hyperforum, it appears to affect mood, although nobody knows precisely how. So far, the most plausible explanation is that it boosts and maintains levels of the feel good hormone serotonin in the brain.

Previous studies have shown supplements of the plant to alleviate mild to moderate depression and in Germany it is prescribed by doctors for mood problems. However, research published in the Journal of the American Medical Association showed it to be no more effective than a placebo for 200 patients with more severe clinical depression.

It also has risks, says pharmacist Helen Marshall. It can interfere with the effectiveness of drugs including cholesterol-lowering medications and the contraceptive pill, and increases the risk of stroke for anyone on blood-thinning drugs or anti-depressants.

"Products vary considerably," warns Linde, and you only need pop into a health food shop to see what he means: capsules, tablets, tinctures, teas and oil-based skin lotions are available. Be careful which you choose. In clinical trials, only products standardised to contain 0.3% hypericin have been proven effective and doses of 300mg are considered optimal. Many do find that cheerfulness is restored after two to four weeks, with none of the side-effects of prescription drugs. To them it is a truly happy pill.