St. John's wort is most commonly used for "the blues" or depression and symptoms that sometimes go along with mood such as nervousness, tiredness, poor appetite, and trouble sleeping. 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 these uses.
An oil can be made from St. John's wort. Some people apply this oil to their skin to treat wounds. Applying St. John's wort directly to the skin might be risky. It may cause serious sensitivity to sunlight.