Antibiotic-Associated Diarrhea

What Is It?

In healthy people, many different species of bacteria live inside the bowel. Many are harmless or even helpful to the body, but a few have the potential to be aggressive troublemakers. Under normal circumstances, the "bad" bacteria are far outnumbered, and the bowel's natural ecological balance keeps them under control. This can change dramatically when a person begins treatment with an antibiotic. This is because antibiotics can kill large numbers of the bowel's normal bacteria, altering the delicate balance among the various species. In most cases, the result is only a mild case of short-term diarrhea that goes away quickly after the antibiotic treatment ends. Occasionally, however, an antibiotic eliminates so many of the bowel's "good" and harmless bacteria that the aggressive "bad" ones are free to multiply out of control.

One type of bacteria in particular, a species called Clostridium difficile (C. difficile), can overgrow inside the bowel, producing irritating chemicals that damage the bowel wall and trigger bowel inflammation, called colitis. This can cause abdominal pain, cramps, diarrhea, and fever. In some cases, high-volume, diarrhea is so frequent that the person develops dehydration (very low levels of body water). A more severe complication of C. difficile overgrowth can lead to a type of bowel inflammation called pseudomembranous colitis. Patients with pseudomembranous colitis are at risk of a severely distended colon that stops functioning (toxic megacolon), which can lead to a hole through the bowel wall (bowel perforation).

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