Though it sounds comically prosaic, St John's wort is a deeply metaphysical plant. Known in the middle ages as Fuga daemonum, or Scare Devil, even its botanical name - Hypericum perforatum - gives a clue to its occult past. Derived from the Greek words hyper (meaning "over") and eikon ("apparition"), it provides a further reminder that the herb was used to vanquish evil spirits and ghosts.
For early Christians, the plant's yellow stamens and bright golden flowers suggested sunlight - proof of its effectiveness against the Prince of Darkness. Because its petals ooze crimson resin when rubbed, the herb is said to have sprung from the earth when St John the Baptist was beheaded. Satan was believed to lose his powers when confronted with it. Gathered on St John's Day (June 24) and hung above doorways and windows, the herb protected those inside. When burned on midsummer bonfires, it frightened supernatural beings away.
Hardly surprisingly, St John's wort was initially used as a treatment for insanity - especially where possession by demons was suspected. By the 17th century, however, it was mainly used to treat wounds. "Boiled in wine and drank, it heals inward hurts or bruises," wrote Nicholas Culpeper in The Complete Herbal. "Made into an ointment, it opens obstructions, dissolves swellings and closes up the lips of wounds."
Since then, herbalists have used the dried tops and flowers of Hypericum perforatum to treat everything from bladder ailments, gout and bronchitis to jaundice, menstrual problems and worms. However, the plant is best known for its calming qualities and the effect it has on conditions such as anxiety, insomnia and depression.
What has triggered this explosion of interest in a common-or-garden weed? As with most herbal bestsellers, the answer lies in society's contradictory attitude towards modern science. While disillusioned with clinically trialled, synthetic antidepressants with their terrible side effects, consumers are nevertheless bowled over by scientific "proof" - based on the same methodology - that herbal remedies work.
In 1996, Dr Klaus Linde reported in the British Medical Journal (BMJ) that St John's wort was as effective as a placebo in treating mild to moderate depression - but with fewer side-effects. A year later, Dr Ernst-Ulrich Vorbach concluded in the journal Pharmacopsychiatry that a double dose of hypericum extract (1800mg a day) was as effective as Tofranil - a brand name for the synthetic drug imipramine - in alleviating severe depression. Then, in September 2000, the BMJ reported a similar study by Dr Helmut Woelk, adding to the already impressive body of research proving the herb was safe.
The trouble is, it may not be. In March 2000, the UK committee on safety of medicines issued a warning about possible interactions between St John's wort and some prescription drugs, including the contraceptive pill. There are worries that the herb may reduce the level of contraceptive in the blood, increasing the risk of pregnancy. "However, the jury is still out," says Michael McIntyre, president of the European Herbal Practitioners Association. "There hasn't been a single case of unwanted pregnancy, though there is a theoretical risk."
Equally worrying, he says, is the way St John's wort may react with cyclosporine - an immune-system drug used in organ transplants. There are also concerns that it may cause high blood pressure, anxiety and confusion in patients taking a class of antidepressants known as MAO inhibitors. "My advice," says McIntyre, "would always be to consult a health professional before taking St John's wort."
Despite these warnings, the herb has an exceptional safety record - and it undoubtedly works. Though the mechanism isn't fully understood, one active ingredient, hypericin, is known to have anti-depressive qualities. However, it is unlikely that it acts alone. Research suggests that a complex combination of essential oils, bioflavanoids and xanthones (all of which have anti-viral and anti-inflammatory qualities) inhibits the enzyme responsible for breaking down the "happiness" chemicals serotonin and norepinephrine in the brain. The result is that the uplifting chemicals remain in the system longer, exerting a mildly euphoric effect.
