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Probiotics are live bacteria (e.g., lactobacilli) commonly found in yogurt and fermented foods such as sauerkraut and kimchi. They may also be found in certain probiotic-fortified foods such as breakfast cereals, energy bars and beverages. Prebiotics are certain nondigestible substances (usually carbohydrates, such as fructo-oliogosaccharides) in foods that are used as fuel by beneficial bacteria already in the body. Sources of prebiotics include asparagus, bananas, garlic, onions, soybeans and breast milk.

Probiotics are beneficial bacteria that can suppress the growth of potentially harmful bacteria in the human body. They are believed to aid digestion and protect against certain gastrointestinal problems (e.g., diarrhea) and possibly other conditions or diseases. They may be especially helpful in stabilizing bacterial levels after the use of antibiotics, which can kill both harmful and beneficial bacteria present in the intestines. 

Probiotics and prebiotics may also be available in supplement form. However, most nutritional experts advise consumers to get their nutrients and other healthful substances, including probiotics and prebiotics, from food sources rather than supplements.

The use of probiotics and prebiotics is generally believed to be safe, although they may not be recommended for people with immune system deficiency, gastrointestinal damage, or for children or pregnant women. A few side effects (e.g., abdominal discomfort, bloating, flatulence) have been reported with the use of certain probiotics and prebiotics.