The reddish substance in the plant's flowers contains numerous biologically active compounds, including hypericin and hyperforin.

Medicinal Claims: People take St. John's wort mostly to relieve symptoms of depression. Study results vary, but there may be a benefit in treating mild to moderate short-term depression. However, a large, well-designed study found that St. John's wort is ineffective in treating severe (major) depression.

St. John's wort has been used in the treatment of vitiligo, but its effectiveness in treating this disorder is unproved.

Possible Side Effects: St. John's wort may cause increased sensitivity to sunlight. Other side effects include digestive tract symptoms, fatigue, and headache. Pregnant women should not take this supplement because it increases muscle tone in the uterus and thus may increase the risk of a miscarriage.

One of the larger problems with St. John's wort is that it may interact negatively with drugs people take (see Medicinal Herbs and Nutraceuticals: Some Possible Medicinal Herb–Drug Interactions). These interactions may lead to toxic reactions or ineffectiveness of the drug.

Last full review/revision February 2009 by Ara DerMarderosian, PhD