Immunotherapy Colitis & Severe Diarrhea Management

This page is for people on checkpoint inhibitor immunotherapy, and for the people who care for them. It explains why diarrhea on these drugs is a call-right-away symptom, what colitis is, and how it is treated.

Call 911 or go to the nearest emergency department for:

• Stools that are black, tarry, sticky, or bloody
• Severe belly pain or tenderness
• Blood in vomit, or vomit that looks like coffee grounds
• Bleeding that will not stop
• Sudden confusion, or being hard to wake

Black, tarry stools are also a sign of bleeding in the gut (MedlinePlus). Do not drive yourself.

Call your care team right away, day or night, for:

• Any diarrhea, even a small change from your normal
• Fever or chills
• Mucus in your stool
• Any other sign of infection, if you also get chemotherapy or have low white blood cells. This includes a sore throat, mouth sores, or burning when you urinate. Do this even if your temperature is normal.

MedlinePlus patient information for checkpoint inhibitor drugs says the same thing in its own way. For pembrolizumab, it lists diarrhea, black, tarry, sticky or bloody stools, and severe belly pain among the signs that need a call to your doctor immediately or emergency treatment. For ipilimumab, it lists diarrhea, bloody or black, tarry, sticky stools, severe stomach pain or tenderness, and fever.

Read what this advice 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.

To see why diarrhea matters so much here, it helps to know what these drugs do. They 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.

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.

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."

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 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.

Colitis is treated differently from ordinary diarrhea. 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.

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.

• 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