Why does chemotherapy cause diarrhea?

Many types of chemotherapy can cause diarrhea. Chemotherapy drugs destroy cancer cells, but they also destroy fast-growing healthy cells. That includes the cells that line your digestive tract.

When those lining cells are damaged, the digestive tract cannot work as smoothly, and diarrhea can result. That is one reason diarrhea is such a common side effect of cancer treatment. It happens with chemotherapy and also with immunotherapy, radiation therapy, targeted therapy, and others.

This matters because severe diarrhea can make your body lose too much water and too many nutrients. That leads to dehydration, which can be life-threatening. So tell your doctor or nurse if you have diarrhea. Your team can look at the cause and the grade, which means how severe it is. They can then suggest fluids, diet changes, or medicines. If chemotherapy is causing severe diarrhea, your doctor may lower your dose or pause it until you feel better. Your health care team can say more about your own treatment.