The single-celled microorganisms known as probiotics are natural inhabitants of the human digestive system. These hundreds of species of bacteria and yeast play a variety of beneficial roles in the body, including synthesizing vitamins and keeping harmful bacteria in check. The medical use of probiotics -- in the form of cultured or fermented dairy products such as yogurt or kefir -- dates to biblical times. But only about 100 years ago were bacteria identified as the healthful components. Some of the species most popular in supplement form today include Saccharomyces yeasts and Bifidobacterium and Lactobacillus bacteria (the best known is L. acidophilus).
Uses: People take probiotics to prevent ulcers, allergies, diarrhea, bacterial vaginosis, yeast infections, tooth decay, the side effects of antibiotics, and to manage lactose intolerance.
Dose: Probiotics are sold as pills, powders, drinks and foods. Recommended doses vary. For vaginal yeast infections, clinical trials suggest 1 to 2 million CFUs (colony forming units) of L. acidophilus a day; For gastrointestinal conditions, 500 milligrams of S. boulardii up to four times a day, or 1 to 2 million CFUs of Lactobacillus or Bifidobacterium daily.
Precautions: Probiotics can cause gas and other digestive discomforts. In people with compromised immune systems, S. boulardii has been linked to a few cases of serious internal fungal infections.