Although St. John's wort (Hypericum perforatum) is a "natural" substance, it can still cause side effects. Nausea, diarrhea, and insomnia are some of the common side effects reported with St. John's wort. This supplement can also cause more serious side effects, such as hallucinations, allergic reactions, and heart palpitations. Notify your healthcare provider right away if you develop these or other serious side effects.
(Click St. John's Wort Side Effects to read the complete eMedTV article, which lists other common side effects of St. John's wort, as well as which side effects require immediate medical attention.)