The herb St John's wort (Hypericum perforatum) has been used for centuries to treat a variety of medical illnesses. In certain areas of Europe, St John's wort has been a commonly prescribed treatment for depression, but, in the United States, it is available for purchase over the counter as an herbal supplement. Some researchers believe that specific chemical constituents of St John's wort produce change in depression in a way similar to that of antidepressant medications, yet this hypothesis is problematic. In addition, studies that support the efficacy of St John's wort in patients with mild-to-moderate depression have limitations that may affect the accuracy of their conclusions. Studies measuring the effect of St John's wort in major depression have reported conflicting results and need to be reexamined. Because St John's wort is considered by some to be an alternative to conventional therapies, clinicians need to know whether it is an effective and safe treatment for different levels of severity of depression. Current evidence does not support its use, and, because of potential drug interactions, St John's wort is not a benign treatment. © Copyright 2009 Physicians Postgraduate Press, Inc.