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St. John's wort is a perennial shrub that grows wild in many parts of the world, including North America and Europe. Flowering in July and August, St. John's wort bears bright yellow blossoms that turn red when bruised. The leaves and flowers are used for health purposes.
The chief active components in St. John's wort are hypericin and pseudohypericin, although other constituents such as xanthones, hyperforin, and flavonoids may also exert biological activity.
The German Commission E Monograph recommends a daily dose of 2 to 4 grams of dried whole herb, or 0.2 to 1 mg total hypericin in other formulations. Many herbalists recommend extracts standardized to 0.3% hypericin content, because that is the concentration used in most research. The standard recommendation for these extracts is 300 mg three times daily with meals. St. John's wort is very well tolerated, with an incidence of adverse side effects similar to that of placebo. The most common adverse effects are gastrointestinal symptoms, dizziness/confusion, and tiredness. In rare cases, it may cause increased sensitivity to sunlight. Because St. John's wort may interact with antidepressant medications or interfere with the action of drugs such as protease inhibitors, contraceptives, transplant rejection drugs, heart disease medications, cancer therapies, and others, people who are taking prescription medications should consult a knowledgeable physician or pharmacist before taking St. John's wort.