Acidophilus is a general name for dried or liquid, live cultures of lactobacillus bacteria, which sour milk and are considered "friendly" organisms in the intestinal tract. You can buy acidophilus products in health food stores. Although most are milk-based, nondairy versions using carrot juice are available. All provide much higher concentrations of the desired organisms than acidophilus milk, yogurt, or other cultured milk products.
I take acidophilus with meals when I travel in underdeveloped countries to reduce the risk of traveler's diarrhea. I strongly recommend taking acidophilus whenever you're on antibiotics, especially the broad spectrum ones like tetracycline and ampicillin, which wreak havoc on intestinal flora. And I recommend it to women who have frequent vaginal yeast infections. The dose is one tablespoon of the liquid culture or one to two capsules after meals unless the label directs otherwise. You also can treat yeast infections by placing the liquid culture directly in the vagina in addition to taking it by mouth. In addition, acidophilus can be helpful to those with chronic intestinal conditions such as irritable bowel syndrome (IBS) and malabsorption syndrome.
I don't think it is possible to take too much acidophilus, but it is possible to waste your money on it. I've seen it recommended for reduction of bad breath and intestinal gas as well as for cholesterol control, enhanced immunity, to lower risk of allergies, treatment of respiratory disorders, prevention of bladder and urinary tract infections, and more. To my knowledge, no solid scientific research exists to back up any of these uses.
You also can waste money on acidophilus products that do not contain viable organisms (check expiration dates) or strains that will survive passage through the strong acid in the stomach. The best products on the market contain a new strain, Lactobacillus GG that has been shown to get into the intestinal tract when you swallow it. One brand to look for is Culturelle (www.culturelle.com).
Andrew Weil, M.D.