St. John's wort is a common perennial herb of many branches and bright yellow flowers that grows wild in much of the world. Its name derives from the herb's tendency to flower around the feast of St. John. (A wort simply means plant in Old English.) The species name perforatum derives from the watermarking of translucent dots that can be seen when the leaf is held up to the sun.

St. John's wort has a long history of use in treating emotional disorders. During the Middle Ages, St. John's wort was popular for "casting out demons." In the 1800s, the herb was classified as a nervine, or a treatment for "nervous disorders." When pharmaceutical antidepressants were invented, German researchers began to look for similar properties in St. John's wort.

Today, St. John's wort is a widely used treatment for depression in Germany, other parts of Europe, and the United States. The evidence-base for its use approaches that of many modern prescription drugs at the time of their first approval.

Most studies of St. John's wort have evaluated individuals with major depression of mild to moderate intensity. This contradictory-sounding language indicates that the level of depression is more severe than simply feeling "blue." However, it is not as severe as the most severe forms of depression. Typical symptoms include depressed mood, lack of energy, sleep problems, anxiety, appetite disturbance, difficulty concentrating, and poor stress tolerance. Irritability can also be a sign of depression.

Taken as a whole, research suggests that St. John's wort is more effective than placebo and approximately as effective as standard drugs. Furthermore, St. John's wort appears to cause fewer side effects than many antidepressants. However, the herb does present one significant safety risk: it interacts harmfully with a great many standard medications. (See Safety Issues for details.)

St. John’s wort has also shown promise for treatment of severe major depression.107,126Note: St. John's wort alone should never be relied on for the treatment of severe depression. If you or a loved one feels suicidal, unable to cope with daily life, paralyzed by anxiety, incapable of getting out of bed, unable to sleep, or uninterested in eating, see a physician at once. Professional care may be lifesaving.

Besides depression, St. John’s wort has also been tried for many other conditions in which prescription antidepressants are thought useful, such as attention deficit disorder,131anxiety, insomnia,15menopausal symptoms,20premenstrual syndrome (PMS),19,102seasonal affective disorder (SAD),98,99and social phobia.103However, there is as yet no convincing evidence that it offers any benefit for these conditions. One substantial double-blind study did find St. John's wort potentially helpful for somatoform disorders (commonly called psychosomatic illnesses).104

Standard antidepressants are also often used for diabetic neuropathyand other forms of neuropathy (nerve pain). However, a small double-blind, placebo-controlled trial failed to find St. John's wort effective for this purpose.16Another study failed to find St. John's wort helpful for obsessive-compulsive disorder.115

St. John’s wort contains, among other ingredients, the substances hypericin and hyperforin. Early reports suggested that St. John's wort or synthetic hypericin might be useful against viruses such as HIV, but these have not panned out.17However, there is some evidence hyperforin may be able to fight certain bacteria, including some that are resistant to antibiotics.18Note: This evidence is far too preliminary to count St. John's wort as an effective antibiotic.

Based on weak evidence that hypericin might have anti-inflammatory properties, St. John’s wort cream has been tried as a treatment for eczema, with some promising results.100

One interesting double-blind study evaluated a combination therapy containing St. John's wort and black cohosh in 301 women with general menopausal symptomsas well as depression.116 The results showed that use of the combination treatment was significantly more effective than placebo for both problems.

In a small placebo-controlled trial, hypericin extract showed no benefit for burning mouth syndrome, a poorly understood condition in which a person experiences ongoing moderate to severe pain in the tongue and/or mouth.129