Acidophilus is a powdered form of a bacteria called 'lactobacillus.' It is used when the intestines have been compromised by illness, stress, or the use of medications like antibiotics. In these situations, the normal 'friendly' bacteria which help in digestion often die and are replaced by bacteria which tend to cause problems such as increased intestinal gas and diarrhea. The lactobacillus bacteria are used to help prevent overgrowth of these undesirable bacteria, and allow the 'friendly' bacteria to recover their normal numbers.