Acidophilus is a powder form of the normal, friendly bacteria that live in our intestines (called flora). Our intestinal flora helps regulate our digestion and stool patterns. Yeast also lives in our intestines. The yeast and the bacterial flora are constantly competing for space, and generally they keep each other in check. Sometimes, however, our bacterial flora can get depleted, such as when taking antibiotics. This allows the yeast to overgrow, which can then lead to a variety of problems. Taking acidophilus replenishes our bacterial flora and reduces overgrow of yeast.