I had a side effect from St. John's Wort -- I got pregnant while I was taking birth control pills. I took my pill at the same time everyday and never missed a day, so I was shocked when I got pregnant. It turns out St. John's Wort interferes with the efficacy of birth control pills. I didn't know that until AFTER I got pregnant. So, ever since then I have made sure that I check for any documented interactions with prescriptions meds I'm taking before I try any herbal supplements.