Lactobacillus acidophilus is a bacteria found naturally in the stomach and intestines. One of the important roles of this bacteria is to prevent the overgrowth of other bacteria in the intestines. It also aids in digestion, reduces dietary cholesterol absorption, and improves immune system function. This bacteria is often used to prevent the diarrhea associated with taking certain antibiotics. Exposure to the broad-spectrum antibiotics can destroy lactobacillus acidophilus, causing diarrhea and yeast infections. Restoring the lost lactobacillus acidophilus has been used to prevent and treat vaginal yeast infections. One or two vaginal suppositories containing 10 million to 1 billion colony forming units have been used once or twice daily to treat vaginal yeast infections. Lactobacillus acidophilus is usually well tolerated; however, it may cause stomach upset in people who are lactose intolerant (upset stomach after eating dairy foods). Lactobacillus acidophilus appears to be safe for use during pregnancy; however, pregnant women should consult their health care provider before using it.