Diarrhoea from chemotherapy or immunotherapy: what to do and when it is an emergency

What should I do about diarrhoea during chemotherapy, and when should I go to the emergency department?

Reviewed by: Equipo CRC Warriors (pendiente de revisión clínica) Last reviewed: 02/11/2026 Updated: 02/11/2026 3 min
In short: Diarrhoea is one of the most common effects of 5-fluorouracil, capecitabine and irinotecan, and it can also appear with immunotherapy, where it is a different thing (colitis) that you should never treat on your own. In most cases it is controlled with loperamide as instructed by your team, fluids and diet; but there are warning signs (many stools, blood, fever, being unable to drink) that are an emergency and cannot wait.

Why it happens

Chemotherapy damages cells that renew quickly, and those lining the bowel are among the fastest. The result is that the bowel absorbs water less well and expels it. The colorectal cancer drugs that most often cause it are:

  • 5-fluorouracil and capecitabine (FOLFOX, FOLFIRI, CAPOX): diarrhoea that usually starts within a few days and worsens if untreated. It is more severe in people with DPD deficiency, which is why this is tested before starting.
  • Irinotecan (FOLFIRI, FOLFOXIRI): two types. Early, during the infusion or in the first 24 hours, with sweating, watery eyes and cramps (cholinergic syndrome; stopped with atropine in the day unit). And late, from day 3-5, which can be severe and prolonged.
  • Anti-EGFR drugs (cetuximab, panitumumab) and some oral drugs (regorafenib, fruquintinib) also cause it frequently.
  • Immunotherapy (pembrolizumab, nivolumab, ipilimumab) can cause immune-mediated colitis: it is a different mechanism, can appear weeks or months after starting, and is not treated with loperamide at home: always tell your team, because it may need corticosteroids.

What to do at home (chemotherapy)

  1. Loperamide as instructed by your team. For irinotecan diarrhoea the regimen is more intensive than the leaflet says (a starting dose and then every few hours until 12 hours have passed without a stool). Do not use it if there is fever or blood without checking first.
  2. Fluids: 2-3 litres a day in sips, alternating water with broths, oral rehydration solution from the pharmacy or drinks containing salts. Water alone does not replace what you lose.
  3. Diet: white rice, pasta, boiled potato, carrot, banana, stewed apple, grilled chicken or fish, toast. Avoid dairy (except yoghurt), fats, fried food, spicy food, coffee, alcohol, raw fruit with skin, raw vegetables, pulses, pastries and sweeteners (sorbitol, "sugar-free" chewing gum).
  4. Small, frequent meals. Look after the skin around the anus: lukewarm water, pat dry, barrier cream.
  5. Note the number of stools per day compared with what is normal for you: that is the figure you will be asked for.

When it is an emergency

Call your day unit's number or go to the emergency department if any of these apply:

  • Seven or more stools a day above your usual, or diarrhoea that wakes you at night.
  • Fever of 38 °C or more (with low white cells, diarrhoea with fever can be a serious infection).
  • Blood or mucus in the stools, or black stools.
  • You cannot keep fluids down; very little, dark urine; dizziness on standing, very dry mouth, cramps.
  • Severe abdominal pain or a swollen, hard belly.
  • The diarrhoea does not improve within 24-48 hours of properly taken loperamide.
  • You are on immunotherapy and have new diarrhoea or a change in bowel habit: report it even if mild.

Dehydration in someone on treatment is serious and is fixed quickly in hospital with a drip. Nobody will criticise you for calling "about diarrhoea".

If you have a stoma

With an ileostomy, diarrhoea means a bag that fills with liquid many times a day, and dehydration arrives sooner. If you have to empty it more than 8-10 times or the output is like water, the same applies: oral rehydration solution, loperamide as prescribed and an early call.

What to ask at your appointment

  • What exactly is my loperamide regimen, and when should I not take it?
  • Which number do I call at night or at the weekend?
  • Have I had the DPD test (and UGT1A1 if I am on irinotecan)?
  • If I am on immunotherapy, what should I do at the first loose stool?
  • Can I take probiotics or a thickening agent? Which ones?

Frequently asked questions

Can I take loperamide on my own?

With chemotherapy, yes, following the regimen your team gave you (which is not the leaflet one in the case of irinotecan). With immunotherapy, no: the diarrhoea may be immune-mediated colitis and loperamide masks it; always report it.

Do probiotics help?

The evidence is limited, and in people with very low white cells some are not recommended. Ask before taking them. Hydration and prescribed loperamide are what really control the diarrhoea.

Does diarrhoea mean my chemotherapy has to be reduced?

If it is severe (grade 3) or sends you to the emergency department, the usual step is to reduce the dose in the next cycle. It is a standard decision that protects your safety without a meaningful loss of effectiveness.

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Sources

This guide is general information prepared by the CRC Warriors Association (non-profit) from public medical sources. It does not replace the assessment of your oncologist or medical team, who know your case. If you spot an error, write to contacto@crcwarriors.org.