The short answer
Diarrhea on a checkpoint inhibitor is a call-now symptom, not a wait-and-see one. Colitis is inflammation of the bowel wall, it is treated with steroids rather than anti-diarrhoea drugs, and it can begin months after your last infusion.
The checkpoint inhibitor instruction is triggered by diarrhea itself, with no number of stools to reach first.
Black, tarry or bloody stools, or severe abdominal pain, mean calling immediately or getting emergency treatment.
Colitis is inflammation of the bowel wall rather than ordinary loose stools, and it is treated differently.
In pembrolizumab labeling, 69% of patients who developed colitis needed systemic corticosteroids.
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The full explanation.
Get help fast if you notice these signs
Call your doctor immediately, or get emergency medical treatment, for any of these signs. The U.S. National Library of Medicine's patient information for checkpoint inhibitor drugs is explicit about this.
For pembrolizumab: "Some side effects can be serious. If you experience any of these symptoms, call your doctor immediately or get emergency medical treatment." The listed symptoms are "diarrhea; stools that are black, tarry, sticky, or contain blood or mucus; or severe abdominal pain."
For ipilimumab, the instruction is the same: "call your doctor immediately or get emergency medical treatment." The listed symptoms are "diarrhea, bloody or black, tarry, sticky stools, severe stomach pain or tenderness, or fever."
Read what this instruction does and does not say. It is triggered by diarrhea itself. It does not ask you to reach a certain number of stools first. It does not tell you to wait and see whether it settles.
What immune checkpoint inhibitors do
These drugs release the brakes on your immune system so it can recognize and attack cancer. The same change can let the immune system attack healthy tissue. The National Cancer Institute (NCI) groups these reactions together as organ-related inflammation. It names each one after the organ involved, using the ending "-itis." When the bowel is affected, it is called colitis.
What colitis is
NCI describes it directly. "Inflammation to organs in your digestive system, called colitis, may cause stomach or abdominal pain, diarrhea, and black or bloody stools."
Colitis is inflammation of the bowel wall itself. It is not simply a change in how food moves through you. It is a different problem from the loose stools that many cancer treatments cause, and it is treated differently.
How it differs from ordinary treatment diarrhea
NCI defines plain diarrhea as "having bowel movements (stools) more often than normal. The stool may also be soft, loose, or watery." Colitis can start out looking exactly like that.
What NCI adds to the colitis description are features that ordinary diarrhea does not usually bring: stomach or abdominal pain, and black or bloody stools.
You cannot reliably tell the two apart at home. You are not expected to. NCI's instruction is clear: "It's important to have any possible inflammatory problems assessed by your oncologist."
What the numbers mean
NCI grades treatment-related diarrhea by how far it exceeds your own normal baseline.
- Grades 1 and 2, meaning up to six bowel movements above your normal daily number, can usually be managed at home.
- Grades 3 and 4, meaning seven or more bowel movements above your normal daily number, can be life-threatening. NCI says these "may require treatment in a hospital."
Those grades come from NCI's general diarrhea guidance, not a checkpoint-inhibitor-specific rule. Treat seven or more above your normal as clearly serious. Do not wait that long to report a problem. Report earlier, per the emergency guidance above.
NCI also notes that your oncologist "may advise you to call him or her first about inflammatory symptoms, or you may be advised to seek emergency medical care." Ask your team which one applies to you before you need it.
Why steroids are used
The problem is an overactive immune response, so treatment targets the immune system rather than the bowel. NCI states it plainly. "Sometimes these side effects are managed with medicines, such as steroids, that work to slow down an overactive immune response."
This is a standard approach, not a sign that something has gone badly wrong. In FDA labeling for pembrolizumab, systemic corticosteroids were required in 69% of patients who developed colitis. The labeled approach is prednisone, or an equivalent steroid, at a strength the team calculates from your weight and the severity of the reaction. It carries on until things have largely settled, then tapers over at least a month. The taper is deliberately slow, and it is not one to cut short by yourself. The drug may be withheld or permanently stopped, depending on severity.
It can start long after your last dose
This is the part people miss. FDA labeling states that "immune-mediated adverse reactions can occur at any time after starting treatment with a PD-1/PD-L1 blocking antibody." They usually appear during treatment, but they "can also manifest after discontinuation."
So if bowel symptoms begin weeks or months after your final infusion, they are still worth reporting. The team who gave you the immunotherapy needs to know. Tell whoever you see — an urgent care clinician, an emergency doctor, or your primary care physician — the name of the immunotherapy drug you received. Tell them when you last had it. That single fact changes how they think about your symptoms.
Sources
- National Cancer Institute — Immunotherapy and Organ-Related Inflammation: https://www.cancer.gov/about-cancer/treatment/side-effects/organ-inflammation
- National Cancer Institute — Diarrhea: https://www.cancer.gov/about-cancer/treatment/side-effects/diarrhea
- MedlinePlus (National Library of Medicine) — Pembrolizumab Injection: https://medlineplus.gov/druginfo/meds/a614048.html
- MedlinePlus (National Library of Medicine) — Ipilimumab Injection: https://medlineplus.gov/druginfo/meds/a611023.html
- U.S. Food and Drug Administration — KEYTRUDA (pembrolizumab) prescribing information: https://www.accessdata.fda.gov/drugsatfda_docs/label/2024/125514s158lbl.pdf
Words to know
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Common questions
When should I call about diarrhea on immunotherapy?
Immediately. The patient information for pembrolizumab and for ipilimumab both say to call your doctor immediately or get emergency medical treatment for diarrhea; stools that are black, tarry, sticky, or contain blood or mucus; or severe abdominal pain or tenderness. Read what that instruction does and does not say. It is triggered by diarrhea itself. It does not ask you to reach a certain number of stools first, and it does not tell you to wait and see whether it settles.
What is immunotherapy colitis?
Checkpoint inhibitors release the brakes on your immune system so it can recognize and attack cancer, and the same change can let the immune system attack healthy tissue. NCI groups these reactions together as organ-related inflammation, naming each after the organ involved. When the bowel is affected it is called colitis: inflammation of the bowel wall itself, which may cause stomach or abdominal pain, diarrhea, and black or bloody stools.
How is it different from ordinary treatment diarrhea?
Colitis can start out looking exactly like ordinary diarrhea, which NCI defines as bowel movements more often than normal, with stool that may be soft, loose or watery. What NCI adds to the colitis description are features ordinary diarrhea does not usually bring: stomach or abdominal pain, and black or bloody stools. You cannot reliably tell the two apart at home, and you are not expected to — NCI says any possible inflammatory problems should be assessed by your oncologist.
Why are steroids used to treat it?
Because the problem is an overactive immune response, so treatment targets the immune system rather than the bowel. NCI says these side effects are sometimes managed with medicines such as steroids that slow down an overactive immune response. In FDA labeling for pembrolizumab, systemic corticosteroids were required in 69% of patients who developed colitis. Prednisone, or something equivalent, is given at a strength your team works out from your weight and how bad the inflammation is, carried on until things have largely settled, then tapered over at least a month.
Can this start after my last infusion?
Yes, and this is the part people miss. FDA labeling states that immune-mediated adverse reactions can occur at any time after starting treatment with a PD-1/PD-L1 blocking antibody, and that although they usually appear during treatment, they can also manifest after discontinuation. So bowel symptoms that begin weeks or months after your final infusion are still worth reporting. Tell whoever you see the name of the immunotherapy drug you received and when you last had it, because that single fact changes how they think about your symptoms.
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Written by: Cancer ExplainedSources last checked: 2026-07-23 what this meansLast updated: 2026-08-19Next planned review: 2027-02-03
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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: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
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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