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 means loose or watery stools that come more often than usual. Some chemotherapy drugs cause it. So can radiation to the abdomen or pelvis, some targeted therapies, and immunotherapy. Infections and certain medicines can cause it too. Diarrhea makes the body lose fluid and salts fast, which 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. Include some that provide salts and a little sugar. Broths, diluted juice, and oral rehydration drinks all help replace electrolytes. Very sugary or caffeinated drinks make diarrhea worse for some people. Call your team if you cannot keep fluids down, or if you feel dizzy and are passing little urine.
Eating while diarrhea lasts
Many people do better for a while with bland, easy-to-digest, lower-fiber foods. Bananas, white rice, applesauce, toast, plain crackers, eggs, and skinless chicken are common choices. 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 sensitive to lactose for a time, so dairy may need to be limited briefly.
About anti-diarrhea medicines
It is safest not to start anti-diarrhea medicine on your own during treatment without checking first. The right approach depends on the cause. If an infection is behind it, slowing the bowel can make things worse. Your team may give you a medicine and clear instructions for when to use it. Everyone's situation is different. This is general information, not advice for you personally. Your care team can tailor it to your treatment, and so can an oncology dietitian if one is available.
When to get help sooner
Diarrhea can dehydrate you within a day. Report it early, even if it feels mild.
- Call 911 or go to an emergency department if you feel faint or confused, your heart is racing, or you cannot stand up without nearly passing out.
- Ring your cancer team immediately, at any hour, if your temperature reaches 100.4°F (38°C) or higher during chemotherapy, with or without diarrhea. CDC treats this as a medical emergency, because infection can move fast when blood counts are low. If you cannot reach them within minutes, go to an emergency department and tell them at the desk that you are on chemotherapy.
- Call 911 or go to an emergency department if severe belly pain comes with fever and diarrhea, or your belly becomes swollen and tender to touch. That combination can mean a serious bowel infection.
- Call your care team the same day if you see blood or black, tarry stool, you cannot keep fluids down, or you have passed little or no urine for eight hours or more.
- Call your care team the same day if you are on immunotherapy and have any new diarrhea. It can be a sign of bowel inflammation that needs treatment quickly.
- Call your care team within a day or two if you have more loose stools a day than usual and it is not settling, or your bottom is becoming raw and sore.
Ask your team in advance how many loose stools in a day should make you call.
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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?
A fever during chemotherapy is a medical emergency: ring your cancer team at once, day or night, and go to an emergency department if you cannot reach them quickly. Contact your team promptly for many loose stools a day, blood in the stool, severe cramping, or signs of dehydration. With some immunotherapies, diarrhea needs prompt attention.
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Sources last checked: 2026-08-13 what this meansLast updated: 2026-08-18Next planned review: 2027-07-13
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Editorial review complete — This page completed Cancer Explained's editorial checks (sources, safety, plain language, duplication). It has not been reviewed by a physician or other healthcare professional.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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