Diarrhea During Cancer Treatment

This page is for people who have diarrhea during cancer treatment. It explains how teams grade it, what causes it, how common remedies compare, and when to get urgent care.

Diarrhea in cancer care is graded on a formal scale called the CTCAE, version 5. The grade matters because it helps decide whether treatment continues, is delayed, or is stopped, and whether you get admitted.

The scale counts stools above your own baseline, not stools in total. This matters if you normally go twice a day or once every two days.

• Grade 1: fewer than 4 extra stools a day over baseline, or a mild rise in ostomy output.
• Grade 2: 4 to 6 extra stools a day, or a moderate rise in ostomy output. Everyday tasks like shopping and cooking get harder.
• Grade 3: 7 or more extra stools a day, hospital admission needed, or a severe rise in ostomy output. Self-care such as bathing and dressing is limited.
• Grade 4: life-threatening, needing urgent intervention.

So it helps to keep a count, not a rough guess. Write down the time of each stool and whether it was loose or watery. A count of "about 8 today, up from my usual 1" gets a very different response than "quite a lot."

The most common trigger is the treatment itself, but different treatments cause it in different ways.

Fluoropyrimidines and irinotecan. Regimens containing 5-FU, capecitabine, or irinotecan cause diarrhea in as many as 50 to 80 percent of people, according to the National Cancer Institute’s “Gastrointestinal Complications (PDQ).” This is direct damage to the gut lining.

Immune checkpoint inhibitors. Here the immune system is attacking the bowel lining, and it can escalate to colitis. Rates depend on the drug class. According to the same NCI source, any-grade diarrhea occurs in:

• 16 to 37 percent of people on a single PD-1 or PD-L1 drug
• 32 to 49 percent on a single anti-CTLA-4 drug
• 17 to 44 percent on a dual checkpoint regimen

Radiation to the abdomen or pelvis. The bowel sits in the treatment field and the lining thins.

Bone marrow transplant. Graft-versus-host disease of the gut is one cause, and it needs its own treatment.

Surgery on the gut. Removing part of the bowel can permanently change how fast food moves through and how the body handles bile.

Infection. Clostridioides difficile (C. difficile), a type of bacteria, is a common cause of pseudomembranous colitis, a severe swelling of the colon, especially after antibiotics. This one matters because it needs its own tests and treatment.

Other causes are easy to miss:

• Tube feeding
• Laxatives that were not adjusted
• Magnesium supplements
• Stool leaking around a blockage

That last one looks like diarrhea and is actually constipation.

Loperamide (Imodium) is an opioid-type medicine that acts on the gut without much effect on the brain. Two dosing patterns appear in cancer guidance, and they are not the same.

General cancer-related diarrhea. The usual pattern is 4 mg to start, then 2 mg after each unformed stool, up to roughly 12 mg per day. Your own prescription is the one that counts, because teams lower this amount for some people.

Irinotecan late diarrhea. The irinotecan label uses a higher regimen: 4 mg at the first loose stool, then 2 mg every 2 hours until you have gone 12 hours with none, and 4 mg every 4 hours overnight. That goes past the usual daily maximum on purpose, and the label warns not to run it beyond 48 consecutive hours because of the risk of paralytic ileus, a stalled bowel.

This is background only. The higher schedule may not apply to you. Ask your care team which one they want you to follow, and get it in writing.

One limit: loperamide is often not enough for grade 3 or 4 diarrhea. If you are past 7 extra stools a day, more loperamide is unlikely to be the answer. Call your care team.

Bulk-forming agents such as psyllium are widely used, but there is little evidence they help here, and the large volume needed often causes bloating.

Adsorbents such as kaolin, clay, and activated charcoal have been widely used without data to support them. Worse, they can block the absorption of the anti-diarrheal drugs that do work.

Bismuth subsalicylate (Pepto-Bismol) is not an option if you cannot take aspirin, and large doses can cause salicylate toxicity.

Octreotide, a hormone-blocking injection, and corticosteroids have specific roles chosen by your team. They are not self-care items.

Replacing lost fluid. Drink lots of water and other clear liquids, such as broth, to replace the fluid and minerals you lose. Ask your team how much to drink. Foods with sodium and potassium, such as soup, bananas and peeled boiled potatoes, can help too.

Food choices. NCI suggests avoiding alcohol, milk and dairy, spicy food, caffeine, high-fat food, fruit juices, and sugar-free gum or candy while symptoms are active. Small, frequent meals are easier than three large ones.

Skin care. Keep the anal area clean and dry. Warm water, baby wipes, or a warm, shallow sitz bath can help.

If your diarrhea began during or shortly after an immune checkpoint inhibitor, say so on the phone and call your team before taking anti-diarrheal medicine. These drugs can inflame the colon (colitis), which needs your team's assessment.

If you have had antibiotics recently, ask whether a stool test for C. difficile should be sent before anything is prescribed to slow the gut.

Call 911 or go to an emergency department for black tarry stool, or a lot of blood in the stool (MedlinePlus). Do not drive yourself.

Call your care team right away, at any hour, for any of these. If you cannot reach your team, or the problem is severe, call 911 or your local emergency number.

• 7 or more stools a day above your normal, or grade 3 by your team's counting
• Any fever at 100.4 F (38 C) or higher alongside diarrhea
• Blood or mucus in the stool
• Passing little or no urine, or feeling dizzy, lightheaded or faint
• Vomiting or nausea so that you cannot keep fluids down
• Belly pain that is severe, constant, or with a swollen rigid abdomen
• Diarrhea not under control within 24 hours
• Diarrhea for the first time during treatment, if your team asked you to report it

Immunotherapy Colitis: What to Ask, Mucositis During Cancer Treatment, Nutrition During Treatment, and Fever During Chemo.

• https://www.cancer.gov/about-cancer/treatment/side-effects/constipation/GI-complications-hp-pdq
• https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=e518dfc6-7e93-4fee-a66c-51e1ab71c056
• https://www.cancer.gov/about-cancer/treatment/side-effects/diarrhea