St. John's wort is a plant native to Europe with yellow, star-shaped flowers. It's often used for depression but can cause serious interactions with some drugs.
St. John's wort is named after John the Baptist. The plant usually begins to flower around the 24th of June, the feast day of St. John the Baptist. It contains many chemicals that act on messengers in the brain that regulate mood.
People most commonly use St. John's wort for depression and mood disorders. There is some strong scientific evidence that it is effective for mild to moderate depression. St. John's wort is also used for symptoms of menopause and many other conditions, but there is no good scientific evidence to support most of these other uses.