Lactobacilli are bacteria that normally live in the human small intestine and vagina. Lactobacillus acidophilus is generally considered to be beneficial because it produces vitamin K, lactase, and anti-microbial substances such as acidolin, acidolphilin, lactocidin, and bacteriocin.

Dosage in adults:
Tablets/Capsules/Liquid/Yogurt: A dose between 1 and 10 billion viable (live) L. acidophilus bacteria taken daily in divided doses is sufficient for most people. Higher doses may cause mild abdominal discomfort, and smaller doses may not be able to establish a stable population in the gut. For vaginal bacterial infections, a dose that has been used is 8 ounces of yogurt containing L. acidophilus in a concentration of 100 million colony-forming units (108CFU) in each milliliter.
Vaginal Suppository:Doses that have been used for vaginal infections include 1 to 2 tablets (containing 10 million to 1 billion CFU in each tablet), inserted into the vagina once or twice daily.

Anal Suppository: Capsules containing 1.5 grams of Lactobacillus acidophilus have been used to treat diarrhea.

Dosage in Children:
Tablets/Capsules/Liquid: Some experts suggest that one-quarter teaspoon or one-quarter capsule of commercially available L. acidophilus may be safe for use in children for the replacement of gut bacteria destroyed by antibiotics. Up to 12 billion lyophilized heat-killed L. acidophilus has been given every 12 hours for up to 5 days. It is often recommended that L. acidophilus supplements be taken two hours after antibiotic doses, because antibiotics may kill L. acidophilus if taken at the same time.

Allergies:
Lactose sensitive people may develop abdominal discomfort from dairy products containing L. acidophilus. 

Side Effects and Warnings :

Although generally believed to be safe with few side effects, oral L. acidophilus should be avoided in people with intestinal damage, a weakened immune system, or with overgrowth of intestinal bacteria.
The most common complaint is abdominal discomfort or gas, which usually resolves with continued use.  Some women have reported burning of the vagina after using L. acidophilus vaginal tablets.

Pregnancy and Breastfeeding :
There is not enough scientific study available to establish safety during pregnancy. Therefore, pregnant women should use L. acidophilus cautiously and under medical supervision.