Lactobacillus acidophilus, commonly referred to simply as acidophilus, is a friendly inhabitant of the gastrointestinal (GI) tract. It, as well as some related strains of bacteria, is known as a probiotic. Probiotic organisms secrete enzymes that support healthy digestion.

They keep the flora of the intestines and vagina balanced and compete with some pathogenic organisms. When the probiotic population of the body is severely decreased, as can occur with treatment by many antibiotics, yeasts and harmful bacteria may take over and cause illness.

Normal and healthy amounts of acidophilus can also be decreased by chronic diarrhea, stress, infections, and poor diet. The species of Lactobacilli that inhabit the GI tract cause an increase of acidity. The bacteria do this by producing lactic acid from milk sugar (lactose).

The increased acidity may promote the absorption of calcium, as well as of some other minerals. Lowered pH also discourages the growth of many pathogenic species of bacteria and yeasts.

The hydrogen peroxide produced by the acidophilus helps to suppress pathogens. Acidophilus may function in the production of some of the B vitamins, such as niacin, pyridoxine, biotin, and folic acid.

Acidophilus is taken by mouth. It is available as powder, liquid, tablets, or capsules, and is also present in some types of milk, kefir, yogurt, and some cheeses. Frozen yogurt does not contain live probiotics. Check product labels to see whether live organisms are present.

The bacteria are killed by pasteurization. Probiotic products are most potent when kept refrigerated. The potency of a given preparation is usually expressed as the number of organisms per capsule. A usual dose of acidophilus is 1–10 billion organisms, divided into three doses per day.

People who are lactose-intolerant may not tolerate acidophilus. The initial use of acidophilus may cause an increase in intestinal gas, which decreases with continued use of the product.

Taking acidophilus in conjunction with some antibiotics, including ampicillin (Amcill, Ampicin) and amoxicillin (Amoxil, Novamoxin), can prevent the diarrhea that is sometimes caused by their use.

One clinical study suggests that acidophilus speeds up the metabolism of sulfasalazine, a medication used to treat ulcerative colitis. The significance of this information is unknown, according to the University of Maryland Medical Center.

Recent evidence suggests that consuming Lactobacillus acidophilus L1 can be effective in lowering blood cholesterol. The February 1999 issue of the Journal of the American College of Nutrition reports on two studies done at the University of Kentucky.

Subjects who consumed the yogurt containing L. acidophilus L1 had cholesterol levels drop by 2.4% in one study and 3.2% in the other. Although the percentages are small, the effect on the risk of heart disease could be significant.

Astudy published in the December 1998 issue of the Brazilian Journal of Medical and Biological Research found that acidophilus induced a nonspecific immune response in experimental mice. Acidophilus is sometimes recommended as an immune booster for people, although as of 2008 the effect has not yet been documented in humans.

Acidophilus may possibly be helpful in the treatment of canker sores, feverblisters, hives, and adolescent acne. Its use has also been suggested as a preventative for colon cancer. Some evidence suggests that acidophilus may reduce the risk of developing an allergic reaction, including asthma, hay fever, and skin reactions, such as eczema.

In fact, some early evidence suggests that if mothers who have at least one relative with asthma, or some other allergy-related illness, take this probiotic while pregnant and breastfeeding, their babies may be less likely to develop asthma.

Clinical studies also have shown acidophilus can help treat respiratory (lung) infections, including sinusitis, bronchitis, and pneumonia, according to the University of Maryland Medical Center.