St. John's Wort has antidepressant properties. However, it interacts with many medications, and if you are already on an antidepressant it raises the risk of serotonin syndrome (which can be life-threatening). Also, natural remedies can have side effects. SJW can cause photosensitivity.

Just because something is natural does not make it inherently better than something artificial. If you are stable on your meds then you've won the lottery and there's no use in changing anything.

Some food for thought: lithium salts can be found in some lakes, morphine comes from opium which comes from a flowering plant, and cyanide is a natural poison.