St John's Wort has long been used in folk medicine to treat depression and sleep disorders. The plant produces a number of different substances that may have anti-depressant properties although it is thought that the whole extract is more effective than its individual constituents. A review of 29 different studies, covering 5,489 participants, whose depression was assesse using the same methodology found that it was superior to placebos and as effective as standard antidepressants with fewer side effects. However, the researchers stressed that people should still see their doctor in the first instance as St John's Wort can interfere with the effectiveness of other medication and not all preparations of St John's Wort are equally effective.
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