St. John's wort is a shrub-like perennial plant that bears bright yellow flowers with five petals. Native to Europe, the United States, and other parts of the world, it grows in rocky soils, fields, and sunny spots. In popular legend, the plant is believed to have the power to ward off ghosts and evil spirits. The herb contains a chemical called hypericin, which may have some effect as a remedy for depression. In Germany, doctors prescribe St. John's wort more often than they prescribe Prozac, the popular antidepressant drug.
Depression results from changes in the levels of brain chemicals that regulate mood.
These chemicals, called neurotransmitters, carry signals from one nerve cell to another. One of the main neurotransmitters responsible for mood is serotonin. Certain prescription antidepressant drugs regulate mood by affecting the serotonin system in various ways. Inside the brain, molecules of serotonin are broken down by chemicals called enzymes. One theory is that hypericin works by blocking the activity of those enzymes, thus keeping the supply of serotonin high.