St. John's wort (Hypericum perforatum) is a plant that has been used as medicine in European countries for centuries. In the United States, it's available as a dietary supplement that some people take for depression, seasonal affective disorder, or other health conditions.

A person who wants to try St. John's wort should first talk to their doctor. Taking St. John's wort may interfere with some medicines, including antidepressants, birth control pills, blood thinners, cancer medicines, and heart medicines. Also, dietary supplements are not regulated in the United States the same way as medicines, so ingredients and quality can be inconsistent.