The short answer
Diarrhea is a common treatment side effect that can lead to dehydration quickly. Replacing fluids and salts, choosing easy-to-digest low-fiber foods for a while, and limiting things that irritate the gut can help. Report diarrhea early — some causes need prompt treatment, and your team can prevent it getting worse.
Chemo, radiation to the belly or pelvis, and some targeted and immunotherapy drugs can cause diarrhea.
Diarrhea can dehydrate you quickly, so replacing fluids and salts is a priority.
Bland, low-fiber foods and smaller amounts are often easier while diarrhea lasts.
Limit greasy, very sweet, high-fiber, caffeinated, and very spicy foods for a while.
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The full explanation.
Why diarrhea happens
Diarrhea — loose or watery, more frequent stools — can be caused by some chemotherapy drugs, radiation to the abdomen or pelvis, some targeted therapies, and immunotherapy. Infections and certain medicines can also cause it. Because diarrhea makes the body lose fluid and salts fast, it can lead to dehydration and weakness, so it is worth acting on and reporting early rather than waiting to see if it settles.
Replacing fluids and salts
The first priority is replacing what you are losing. Sip fluids steadily through the day, and include some that provide salts and a little sugar — broths, diluted juice, or an oral rehydration drink can help replace electrolytes. Very sugary or caffeinated drinks can make diarrhea worse for some people. If you cannot keep fluids down or feel dizzy and are passing little urine, that is a reason to call your team.
Eating while diarrhea lasts
Many people do better for a while with bland, easy-to-digest, lower-fiber foods — things like bananas, white rice, applesauce, toast, plain crackers, eggs, and skinless chicken. Eating smaller amounts more often is usually gentler than large meals. It often helps to limit greasy or fried foods, very sweet foods, high-fiber and raw foods, caffeine, alcohol, and very spicy dishes until things settle. Some people become temporarily sensitive to lactose, so dairy may need to be limited briefly.
When to call, and about medicines
Tell your team about diarrhea early. Contact them promptly for many loose stools in a day, diarrhea with fever, blood in the stool, severe cramping, or signs of dehydration. With some immunotherapies in particular, diarrhea needs prompt attention. It is safest not to start anti-diarrhea medicine on your own during treatment without checking, because the right approach depends on the cause. Everyone's situation is different. This is general information, not advice for you personally — your care team, and an oncology dietitian if one is available, can tailor it to your treatment.
Words to know
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Common questions
▸What causes diarrhea during treatment?
Some chemo drugs, radiation to the belly or pelvis, some targeted and immunotherapy drugs, infections, and certain medicines. Report it so your team can find the cause.
▸What should I eat with diarrhea?
Many people do better on bland, low-fiber foods for a while — bananas, rice, applesauce, toast, eggs, skinless chicken — in smaller, more frequent amounts.
▸What should I limit?
Greasy or fried foods, very sweet foods, high-fiber and raw foods, caffeine, alcohol, and very spicy dishes until things settle. Some people limit dairy briefly.
▸When is diarrhea an emergency?
Call your team promptly for many loose stools a day, fever, blood in the stool, severe cramping, or signs of dehydration. With some immunotherapies, diarrhea needs prompt attention.
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