St. John's Wort (Hypericum perforatum) caplets. St. John's wort is a perennial native to many parts of the world including Europe and the United States, growing wild in northern California, southern Oregon and Colorado. Historically used as a nerve tonic, St. John's wort is now widely used as a mild antidepressant. It's potent antiviral and antibacterial properties are being investigated as a treatment for AIDS.