St. John's wort (Hypericum perforatum) has a history of use as a medicine dating back to ancient Greece, where it was used for a range of illnesses, including various nervous disorders. St. John's wort also has antibacterial, antioxidant, and antiviral properties. Because of its anti-inflammatory properties, it has been applied to the skin to help heal wounds and burns. St. John's wort is one of the most commonly purchased herbal products in the United States.

In recent years, St. John's wort has been studied extensively as a treatment for depression. Most studies show that St. John's wort may help treat mild-to-moderate depression, and has fewer side effects than most other prescription antidepressants. But it interacts with a number of medications, so it should be taken only under the guidance of a health care provider.