Side effects of St. John's wort include headache, muscle cramps, anxiety, restlessness and irritability, states WebMD. Stomach upset, fatigue, dry mouth, dizziness and skin rash may also occur. Taking St. John's wort in the morning or decreasing the dose may help people who experience trouble sleeping while taking the supplement.
Taking St. John's wort may affect other medications by decreasing their effectiveness or altering their levels in the blood, according to Mayo Clinic. Individuals with certain mental illnesses may experience mania or psychosis while taking St. John's wort. The supplement may also increase the skin's sensitivity to the sun, cause changes in menstrual flow and alter hormone levels. Liver damage, blood pressure changes, abnormal heart rhythms and swelling are other potential side effects of St. John's wort. Individuals with diabetes should use caution when taking St. John's wort, as the supplement sometimes changes the way the body processes sugar.
As of 2015, early research suggests that St. John's wort may be an effective treatment for anxiety disorders, according to Mayo Clinic. Based on human studies, St. John's wort is as effective as some prescription antidepressants for short-term treatment of mild to moderate depression. Patients should consult their health care provider before taking St. John's wort to treat any medical condition.