There are many types of bacteria and yeast that normally live on our skin, in our intestines, and on other body surfaces. They’re known as our “normal flora.” When the normal flora is in balance, these bacteria do not invade beneath the surface that they live on. They do not disrupt our normal body functions. And they do not make or give off toxic substances that could harm us.

With widespread use of antibiotics, some of the “good bacteria” that live in our intestines is killed. This changes our normal flora. This change in our normal flora can lead to a chemically different environment in the gut or other body area. And that can cause a few problems.

One problem is colitis. It’s an infection that can be caused by the bacteria Clostridium difficile (“C. diff”). It is normal to have some spores from this bacteria in your intestine. When your normal flora is varied and healthy, C. diff stays in its spore form. But changed chemistry in the gut can allow C. diff to change from spores into more active bacteria. When this happens, C. diff makes a toxin that can cause diarrhea and life-threatening inflammation of the colon.

Another problem, especially for women, is yeast infection. It is normal for women to have some yeast in the vagina. But altered chemistry in the vagina from a change in normal flora can cause yeast to multiply and change shape. When this happens, yeast begin to make glue-like proteins that can attach to the surface of the vagina and form clumps. Yeast in this changed form irritate and inflame the vagina, causing vaginitis.

Probiotics are pills, yogurts or other preparations that contain bacteria found in the healthy normal flora of the gut. The idea behind probiotics is to “boost” the populations of bacteria that seem to be present in good health.

Several small studies have shown benefit from specific probiotics. These studies have suggested that probiotics might protect against children’s diarrhea, adult Clostridium difficile diarrhea after antibiotic use, pancreatitis complications and vaginal yeast infections. There have also been studies that showed a possible trend towards less childhood allergy (specifically, eczema rashes) when nursing mothers took in normal flora bacteria. One study showed that probiotic drops with Lactobacillus reuteri bacteria led to less crying in very young nursing babies. So it’s possible that this probiotic may ease colic pains.

I am not aware of any harm of using the probiotics being marketed today. Yogurt is also a healthy food, and it makes sense to include it in your diet. But most people do not need probiotics. And I do not recommend them for people who are already in good health. The most important way to keep your normal flora in balance is to only take antibiotics when necessary.