My IBS flared up like mad when I got on birth control pills. My doctor and I even thought that's what was causing my upset stomach. I was on Ortho-Tri-Cyclen, but switched to Alesse due to the stomach problems, and they continued, so I went back to Ortho Tri-Cyclen. I've been on them for....almost 3 years. It'll be three years in October. And I've been sexually active the entire time (with the exception of the first couple of months) and have never become pregnant. Though, I have very mild IBS, and I also take my pill right before I go to bed at night. Around 10 or 11. I don't know if that makes a difference or not.I, too, have always wondered about that though.... Because sometimes I'd have diarrhea everyday for a week.....But I never got pregnant. So, I've either been insanely lucky, or it never lost it's effectiveness.