Antibiotic-associated colitis is an irritation in your large intestine. It happens when there is a disruption in the bacteria of your intestines after taking antibiotic medication. Colitis can lead to diarrhea and abdominal cramping.

The infection is often very serious.

The Stomach, Liver, and Intestines
Normal Anatomy of the Large and Small Intestine
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Your intestine is normally full of good bacteria. When you take antibiotics, they often kill all the good bacteria in your intestine. This creates a perfect home for bacteria called Clostridium difficile ( C. diff) . This particular bacteria is not killed by the antibiotics and begins to grow out of control. As it grows, the bacteria makes toxins. These toxins irritate the lining of the intestine and cause swelling, leading to pain and diarrhea.