context: Probiotics is a general term for living microorganisms -- often called "friendly" bacteria -- that have health benefits in the body. These can be bacteria or yeast that are similar to organisms that are naturally found in the body, especially in the digestive tract. Probiotics have become popular supplements and food additives, most often used to promote healthy digestion. Probiotics work by balancing the levels of microorganisms in the intestines. They drive down the numbers of harmful bacteria. They also seem to boost the body's immune system. Although research is ongoing, there's good evidence that some probiotics may be helpful in treating irritable bowel syndrome, some types of diarrhea, colitis (particularly ulcerative colitis and and a complication of surgery for ulcerative colitis called pouchitis), acne, and eczema in children. They may also be used with antibiotics to help prevent diarrhea that may come with taking antibiotics. In addition, researchers are studying probiotics to determine if they may help certain types of stomach ulcers (those caused by H. pylori), infections (including urinary tract, vaginal, GI, sinus, and respiratory), dental disease, allergies, and diseases of the liver. However, more research is needed to determine if probiotics are safe and effective for these conditions. There are many types of probiotics. They include lactobacilli (like Lactobacillus acidophilusand Lactobacillus GG), bifidobacteria (like Bifidobacterium bifidus) and some yeasts (like Saccharomyces boulardii). Different probiotics have different effects. So while one may help treat diarrhea or a vaginal infection, another may have no effect. Before you start taking a probiotic supplement, talk to your health care provider to make sure that you get the treatment most likely to help. Probiotics are different from prebiotics. Prebiotics are non-digestible ingredients in foods that are used to spur the growth of probiotic bacteria in the body by providing a suitable environment in which the probiotics themselves can flourish. Synbiotics are combinations of prebiotics with probiotics. Because there are so many different probiotic organisms, there is no set dosage. Ask your health care provider for advice. Some probiotics are dosed by the number of live organisms they contain. For instance, a typical dosage of Lactobacillus acidophilus ranges between 1 billion to 10 billion live organisms split into three or four doses. Dosage may also be indicated as colony forming units ( CFU). Probiotics occur naturally in some foods and are added to others. Examples are yogurt, kefir, sauerkraut, tempeh, miso, soy drinks, and some other beverages. Side effects. Probiotics seem to have few side effects. Some may cause intestinal gas and bloating. However, this is likely to get better over time. If your probiotics are causing these side effects, try decreasing the dose or using it every other day. Interactions. If you have any medical problems or take any medicines regularly, talk to your health care provider before you start using probiotics. They could interact with medicines such as antibiotics or immunosuppressive drugs. Risks. If you have intestinal disease or damage, HIV, cancer, a weakened immune system, or excessive bacteria in your intestines, don't use probiotics without checking first with your health care provider. Lactobacillus and bifidobacteria are two of the most common types of probiotics on the market.