Managing Diarrhea & Dehydration During Cancer Treatment

The National Cancer Institute (NCI) defines it simply: "Diarrhea means having bowel movements (stools) more often than normal. The stool may also be soft, loose, or watery."

The phrase that matters is than normal. Everything below is measured against your own usual pattern, not against someone else's. If you normally go once a day, four times a day is three above your baseline. Knowing your own starting number before treatment begins makes every later conversation faster.

NCI lists the causes as certain cancers, chemotherapy, immunotherapy, radiation therapy, surgery, targeted therapy, bone marrow or stem cell transplant, stress and anxiety, medicines, supplements, and infections.

Chemotherapy damages the fast-dividing healthy cells that line the digestive tract, which is why loose stools are so common. The American Cancer Society names 5-fluorouracil, capecitabine, and irinotecan among the drugs most associated with it.

Radiation causes it when aimed at the abdomen, pelvis, or rectum, because healthy bowel tissue in the field is affected along with the target.

Targeted therapy commonly causes diarrhea; the American Cancer Society singles out tyrosine kinase inhibitors.

Surgery involving the esophagus, stomach, gallbladder, or bowel can change how food moves and how it is absorbed.

Infection is a real possibility rather than an afterthought. Viral, bacterial, and foodborne infections are more likely when your immune system is weakened by treatment, and the American Cancer Society specifically names C. difficile and neutropenic enterocolitis. Transplant patients can also develop graft-versus-host disease.

Medicines and supplements count too — antibiotics, anti-inflammatory drugs, and some treatments for diabetes and depression, plus herbal products.

Because the list is this long, the cause is not something you can work out at home. Your team needs to know it is happening so they can find out which one is driving it.

Checkpoint inhibitor drugs can inflame the bowel wall itself, a condition called colitis, and it is managed differently from the diarrhea described here. If you are receiving immunotherapy, read Immunotherapy Colitis & Severe Diarrhea Management as well, and follow the reporting rule there rather than this one.

NCI states it directly: "Grades 1 and 2 (having up to six bowel movements above your normal daily number) can usually be managed at home, but grades 3 and 4 (having seven or more bowel movements above your normal daily number) can be life-threatening and may require treatment in a hospital."

Seven or more above your normal daily number is the line NCI draws. Note that it is above your normal, not a total.

Severe diarrhea keeps your body from absorbing enough water and nutrients, and you lose electrolytes and minerals along with the fluid. NCI's warning is short: "Dehydration can be life-threatening, so tell your doctor or nurse if you have diarrhea."

MedlinePlus lists the adult signs of dehydration as feeling very thirsty, dry mouth, urinating and sweating less than usual, dark-colored urine, dry skin, feeling tired, and dizziness.

It also lists the signs of severe dehydration — confusion, fainting, lack of urination, rapid heartbeat, rapid breathing, and shock — and says to "get medical help right away" if symptoms include these.

Tell your doctor if you have:

• Blood or mucus in stools
• Dizziness or lightheadedness
• Fever
• Weight loss

The American Cancer Society adds that you should call 911 for bleeding from your rectum that will not stop, sudden intense belly pain that will not stop, or being unable to urinate or eat for a day or more.

• Drink lots of water or other fluids. Clear liquids such as water or broth help replace lost fluids and electrolytes, and room temperature liquids are easiest on the stomach.
• Eat frequent small meals instead of three larger ones.
• Choose low-fiber foods such as white bread, pasta, and canned fruit.
• Include foods with sodium and potassium: potatoes, soup, bananas, applesauce, crackers.
• Avoid alcohol, dairy, spicy foods, caffeine, dried beans, high-fat foods, fruit juices, and sugar-free gum.
• Keep the anal area clean and dry with warm water and baby wipes; sitz baths can help.
• Track your bowel movements so you can give your team a number rather than an impression.

• What symptoms or problems should I call you about?
• What medicines can I take for diarrhea?
• What can help decrease rectal pain and irritation?
• How much and what types of liquid should I drink each day?
• What foods should I eat while I have diarrhea? What foods should I avoid?
• Could I meet with a registered dietitian to learn more?

• National Cancer Institute — Diarrhea: https://www.cancer.gov/about-cancer/treatment/side-effects/diarrhea
• MedlinePlus (National Library of Medicine) — Dehydration: https://medlineplus.gov/dehydration.html
• American Cancer Society — Diarrhea: https://www.cancer.org/cancer/managing-cancer/side-effects/stool-or-urine-changes/diarrhea.html

---

<script type="application/ld+json"> { "@context": "https://schema.org", "@type": "MedicalWebPage", "headline": "Managing Diarrhea & Dehydration During Cancer Treatment", "description": "What causes diarrhea during chemotherapy, radiation, targeted therapy and surgery, how severity is graded against your own baseline, the signs of dehydration, and what to call about.", "url": "https://cancerexplained.org/side-effects/severe-diarrhea-during-cancer-treatment", "datePublished": "2026-07-23", "dateModified": "2026-07-28", "publisher": { "@type": "Organization", "name": "Cancer Explained", "url": "https://cancerexplained.org" }, "author": { "@type": "Organization", "name": "Cancer Explained Editorial Team" }, "medicalAudience": "Patient", "aspect": "Cancer Information and Support" } </script>