According to WebMD, St. John's wort is a plant with yellow flowers that has been used for hundreds of years by European people in the treatment of mild to moderate depression. St. John's wort is sold as a dietary supplement in the United States.
According to Drugs.com, St. John's wort may also be used in the treatment of nervousness, insomnia and infections. St. John's wort is thought to enhance mood by balancing chemicals in the brain. According to the Mayo Clinic, there is not sufficient evidence to determine whether or not this herbal product is effective with severe depression. People should consult with their doctor prior to beginning a regimen with this product, as St. John's wort may have serious interactions with prescription drugs, herbs or supplements.