In general St. Johns Wort is well tolerated by most people taking it. There are a few who experience minor side effects such as drowsiness, confusion and mild headaches. A very small portion of users will experience an increased sensitivity to sun light which is known as photo-sensitivity. Those suffering from this condition can get sunburned in low amounts of sun exposure or experience difficulty seeing in bright light. St. Johns Wort can also effect the function of certain types of birth control.
As always talk to your doctor before taking any supplement as it can interfere with prescription medicines. Always make sure to only use the recommended dosage of St. Johns Wort.