Why does chemotherapy cause diarrhea?

Many types of chemotherapy can cause diarrhea. This happens because chemotherapy drugs destroy not only cancer cells but also rapidly dividing healthy cells, including the cells that line your digestive tract.

When these 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 many cancer treatments, not just chemotherapy but also immunotherapy, radiation therapy, targeted therapy, and others.

It matters because severe diarrhea can cause your body to lose too much water and too many nutrients, leading to dehydration, which can be life-threatening. For that reason, it is important to tell your doctor or nurse if you have diarrhea. Your team can look at the cause and grade (severity) and recommend 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 healthcare team can say more about your specific treatment.