Immune colitis — what it is and how it's treated

Immune colitis is inflammation of the colon lining caused by checkpoint inhibitor immunotherapy waking up the immune system against healthy gut tissue, not by an infection or anything you ate. It usually shows up as loose or watery stools — sometimes with blood or mucus, cramping, or belly pain — and can start after any cycle, including months into treatment.

This page is the clinical companion to our symptom-triage guide on loose motions during immunotherapy. If you're still deciding how urgent your symptoms are right now, use the checklist above first. This page explains what immune colitis actually is, how doctors confirm it, and what happens to your immunotherapy afterwards.

Families often dismiss early gut symptoms as an ordinary stomach bug — the single biggest reason immune colitis becomes dangerous is that it gets treated like food poisoning for too long before anyone asks whether it could be immune-related.

Grading follows NCCN and ASCO immune-related adverse event guidelines. Only a doctor examining you can confirm the grade — use this table to understand the pattern, not to self-diagnose.

1. 1

   Stool tests to rule out infection

   Stool culture, parasite screening, and a C. difficile toxin test come first, because an infection is treated completely differently from an immune reaction.

2. 2

   Blood tests

   Inflammation markers, electrolytes, and hydration status are checked, since colitis can cause fluid and salt losses even before it's confirmed.

3. 3

   CT scan of the abdomen, if needed

   Used to check the extent of inflammation and rule out complications, particularly in more severe presentations.

4. 4

   Colonoscopy or sigmoidoscopy with biopsy

   A camera examination with a small tissue sample confirms immune colitis directly and grades how severe it is, guiding the treatment plan.

Not always — and this is decided case by case, not by a fixed rule. Three things matter most: how severe the colitis was, how completely it resolved, and which type of immunotherapy you're on.

• Grade 1 that resolves quickly — immunotherapy often continues on schedule with closer monitoring.
• Grade 2 that fully resolves on corticosteroids — restarting a PD-1 or PD-L1 inhibitor is sometimes possible, decided by your tumour board.
• Grade 3–4, or colitis on combination immunotherapy — more often leads to permanently stopping that particular drug, especially the CTLA-4 component.
• Not restarting is a real option — if the cancer is well controlled and the risk of recurrence is judged too high, your team may recommend stopping for good rather than restarting.

There is no single right answer here. Your oncologist and gastroenterologist weigh your cancer control against the risk of the colitis returning, and talk through the trade-offs with you directly — this is exactly the kind of decision a tumour board exists for.

• Counting Stools: The Simple Rule That Decides Urgency — the numeric rule that tells you when a stool-count change needs a call.
• Blood or Mucus in the Stool During Immunotherapy — why this specific sign is never considered normal, and what it means.
• Immune Hepatitis: Liver Inflammation From Immunotherapy — the liver counterpart to gut irAEs, often found on routine blood tests rather than symptoms.
• Immunotherapy at CION Cancer Clinics — the full picture of how CION supports patients through immunotherapy and its side effects.

This page is for general information and does not replace a consultation. If you develop bloody stools, severe abdominal pain, fever, or signs of dehydration while on immunotherapy, call now or go to the nearest emergency department.

Thousands have navigated immunotherapy side effects with us

Gut symptoms during immunotherapy are common, and most are manageable when caught early. Take the first step today — our team walks this part of the journey with you.