As with most herbal supplements, it has not been approved by the FDA so its effects may be relatively unknown or even dangerous. Remember that the supplement makers are not required by law to warn consumers about the health risks of their products. A recent study by the National Center for Complementary and Integrative Health (NIH) found that St. John's wort is not effective in treating major depression.
If you still want to try this supplement, talk about it with your doctor so they can explore how it will interact with other medications you may be taking.