St. John's Wort is an herbal SSRi. It is sold commercial as an herbal supplement to treat clinical depression. You should see your doctor before using it to treat severe depression.
St. John's Wort needs to be mixed carefully, because it is an SSRi (like Kanna). It will potentiate many herbs (probably the same herbs that are potentiated by kanna). It does not mix with many medications, and will depotentiate almost all psychedelics.
Because it is a powerful potentiator it is used in many blends (like Cloud9 from HS).
Tell your doctor you are taking St. John's Wort when he prescribes you a medication.