Fiber? Brewers Yeast? No, Culturelle contains no yeast or sugar or dairy or anything but a bacteria called Lactobacillus GG (named after two doctors named Goldin and something else beginnning with "G", who first isolated the strain). And some sodium and some gelatin, I think (I have to wait until my box arrives). I don't know if it's helpful for IBS -- no-one on here that I know except this recent poster found it helpful for their IBS, frankly, but obviously there is a doctor out there who believes in it and a patient who improved on it.Where it usually comes in is, say, if you have to take an antibiotic or are traveling somewhere where you might get food or water poisoning. It differs from other probiotics in that it survives stomach acids and actually can survive the blast of an antibiotic, arming the intestines like a little barrier, staying in the tract for up to seven days. The ordinary probiotic is blasted out with the first dose and you have to keep taking it. There IS another probiotic -- I think it's Bifidia Bacterium, which is in PB8 and other good probiotics -- that survives stomach acid, but that's all it does. But Culturelle, as I said, stays in the intestinal tract longer. Its usually marketed for travelers diarrhea, taken a few days ahead of time as a preventative. Also, studies have shown it can actually cure recurring relapses of C Difficile, that awful overgrowth caused by strong or too many antibiotics. So maybe there's a lot about it that we don't know.