Why do immunotherapy side effects cause a change in bowel habits?

Immunotherapy helps the immune system recognise cancer, but immune activity can sometimes affect healthy bowel tissue. Inflammation may speed movement through the bowel, producing loose stools or diarrhoea. It can also cause cramps, urgency, mucus, or blood. Other medicines, infection, diet changes, and constipation from treatment can cause similar changes.

A new bowel pattern during or after immunotherapy deserves attention because bowel inflammation can worsen even when the first change seems mild. The timing, frequency, stool appearance, pain, hydration, and other medicines help the care team decide what evaluation is needed.

Record each bowel movement, its consistency, urgency, and any blood or mucus. Tell your oncology team when the change began and whether you have cramps, fever, dizziness, or reduced urination. Drink fluids as advised and choose foods that your care team says are suitable.

Do not start anti-diarrhoeal medicines, laxatives, or supplements without checking first, because the right choice depends on the suspected cause. Your team may assess hydration, review medicines, and arrange stool or blood tests. They may adjust immunotherapy or prescribe treatment if immune inflammation is suspected.

Contact your oncology team promptly for new diarrhoea, repeated urgency, persistent constipation with pain, blood or mucus in stool, fever, or abdominal tenderness. Seek urgent help for severe abdominal pain, fainting, confusion, inability to keep fluids down, or signs of dehydration such as very little urine.

Ask whether the change could reflect bowel inflammation, infection, or another medicine, what to eat and drink, which medicines to avoid, and exactly when to call the treatment team.