The Medical Letter on Drugs and Therapeutics
Probiotics Revisited
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Probiotics are live, nonpathogenic microorganisms (usually bacteria or yeasts) marketed as dietary supplements. They have not been approved by the FDA for any indication. Since our last article on this subject,1 some new data have become available.

MECHANISM OF ACTION — Several mechanisms have been proposed to explain how probiotics could have beneficial effects. Saccharomyces boulardii, a strain of the yeast S. cerevisiae, has been shown to inhibit the pathogenicity of bacterial toxins.2 Acetic, lactic and propionic acid produced by Lactobacillus spp. could lower intestinal pH and inhibit growth of pathogenic bacteria such as Escherichia coli and Clostridium spp. The presence of Lactobacillus spp. and other probiotics in the intestinal tract might physically or chemically prevent adhesion and colonization of pathogenic bacteria. They may also induce or enhance an immune response.

SOURCES — Probiotics can be found in various foods and dietary supplements. Yogurt is probably the most familiar source; according to the FDA, it must be made with Lactobacillus bulgaricus and Streptococcus thermophilus to be called yogurt in the US. Probiotics are also available as dietary supplements in various powders, liquids and capsules; the quantity and quality of the bacteria or yeast and the purity of these products are uncertain.

CLINICAL STUDIES — Studies of probiotics for treatment of gastrointestinal disorders, including some recent randomized controlled trials, are listed in a table on our web site ( In addition to these, a recent meta-analysis concluded that moderate-quality evidence suggests that probiotic prophylaxis results in a large reduction in Clostridium difficile-associated diarrhea.3 Probiotics have also been used in the treatment of many other conditions, including atopic dermatitis, lactose intolerance, bacterial vaginosis, allergic rhinitis, asthma, and peptic ulcer associated with Helicobacter pylori infection.4,5

ADVERSE EFFECTS — Probiotics can cause gas, diarrhea, bloating and hiccups; these are usually mild and transient. Infectious complications have occurred with specific probiotics, including sepsis with Lactobacillus casei, fungemia with S. boulardii, and liver abscess due to Lactobacillus GG, all in highly immunosuppressed and/or critically ill patients, particularly those with an indwelling vascular catheter.6

DRUG INTERACTIONS — Antibiotics can inactivate bacteria-derived probiotics. Florastor, which contains S. boulardii, should not be taken with oral systemic antifungal medications, such as fluconazole.

CONCLUSION — The results of a few randomized controlled trials suggest that probiotics such as Lactobacillus spp. and Saccharomyces boulardii may be helpful in some gastrointestinal disorders, such as Clostridium difficile infection and antibiotic-associated diarrhea. While most probiotics have few, if any, adverse effects in otherwise healthy people, they have caused serious infections in some highly immunosuppressed or critically ill patients.

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