Diarrhoea During Chemotherapy: — What to Do
Diarrhoea during chemotherapy is common, but it is not always minor. At certain points in your treatment cycle it can become serious very quickly. Knowing when to act — and how fast — is the single most useful thing on this page.
Medically reviewed by Dr. Owais Mohammed, Medical Oncologist · Last reviewed September 2026
- Not just an inconvenience — During the nadir, diarrhoea can allow bacteria to cross from your bowel into your bloodstream.
- Same-day contact is the safe default — Call your oncology team the same day diarrhoea starts — not at your next scheduled appointment.
- Do not self-medicate during the nadir — Anti-diarrhoeal tablets can mask worsening that your team needs to know about.
- Fluids matter, but not just water — Oral rehydration solution replaces electrolytes that plain water does not.
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Diarrhoea is a common side effect of chemotherapy, but it is not always minor. During the nadir — the days when your white blood cells are at their lowest — loose motions can raise the risk of serious infection. Call your oncology team the same day. Do not wait to see if it settles.
Why does chemotherapy cause diarrhoea?
Chemotherapy targets cells that divide rapidly — which includes cancer cells, but also the lining of your gut.
The gut lining normally replaces itself every few days. When chemotherapy disrupts that process, the lining becomes inflamed and damaged.
Fluid that your gut would normally absorb passes through instead. The result is loose, frequent motions.
This is a direct effect of treatment on healthy tissue. It is not a sign that something unexpected has gone wrong.
When does diarrhoea become dangerous during chemotherapy?
The most dangerous period is the nadir — the days after each cycle when your white blood cells drop to their lowest level.
During the nadir, bacteria that normally stay inside the bowel can cross through the damaged gut lining into the bloodstream.
Diarrhoea at this point can trigger a serious infection very quickly. It is not an inconvenience to manage at home.
NCCN and ASCO guidance treats any new or worsening diarrhoea during chemotherapy as a symptom that needs same-day assessment, not wait-and-watch.
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What is the nadir in chemotherapy?
- Nadir
- The days after each chemotherapy cycle when your white blood cell count is at its lowest. Your immune defences are weakest during this window, which is why symptoms such as diarrhoea carry more risk at this time than at other points in the cycle.
- Gut mucositis
- Inflammation and damage to the inner lining of the bowel caused by chemotherapy. It is the main reason chemotherapy causes diarrhoea, and it is also why the gut becomes more vulnerable to infection during treatment.
- Electrolytes
- Salts your body depends on to function — including sodium and potassium — that are lost in large quantities during diarrhoea. Replacing them with oral rehydration solution is more effective than drinking plain water alone.
- ORS (Oral Rehydration Solution)
- A drink formulated to replace both fluid and electrolytes. Available at most pharmacies in India as sachets — brands such as Electral are widely stocked. Your team may recommend this over plain water or sports drinks during diarrhoea.
What should you do if you have diarrhoea during chemotherapy?
Call your oncology team the same day. Tell them how often you have been to the toilet, whether there is blood or mucus in your stool, and whether you have a fever or feel lightheaded.
Keep drinking fluids while you wait. Oral rehydration solution — available as sachets at most pharmacies — replaces electrolytes that plain water does not.
Do not take anti-diarrhoeal tablets such as loperamide without asking your team first. During the nadir, these medicines can mask a worsening that your team needs to know about.
If you cannot reach your team and you have a fever, blood in your stool, or cannot keep fluids down, go to the nearest emergency department rather than waiting.
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Frequently asked questions
Can I take loperamide for diarrhoea during chemotherapy?
Not without asking your oncology team first, and particularly not during the nadir. Loperamide settles the symptom while the underlying gut inflammation continues, and during the nadir it can mask signs of serious infection that your team is watching for. If your team has already told you it is safe for you to use, follow their guidance. Otherwise, call before taking anything.
Is the diarrhoea caused by chemotherapy or could it be something else?
Usually chemotherapy is the cause, but not always. An infection, a change in diet, anxiety, or other medicines including antibiotics can all cause loose motions during treatment. Your team will ask questions to work out the likely cause, and the assessment may include blood tests. This is one of the reasons it matters to call rather than assume — the cause changes what the right response is.
What should I drink if I have diarrhoea during chemotherapy?
Oral rehydration solution is a better choice than plain water alone. Diarrhoea causes you to lose electrolytes — particularly sodium and potassium — that water does not replace. ORS sachets such as Electral are available at most pharmacies in India and should be mixed with boiled and cooled water. Avoid sweet drinks, fruit juices and tea, which can make diarrhoea worse. Your team will advise on amounts based on how often you are going.
What if I cannot reach my oncology team?
If you have a fever alongside diarrhoea, blood or mucus in your stool, or you cannot keep fluids down, go to the nearest emergency department rather than waiting. If you feel unwell in any way during the nadir and cannot reach your team, hospital assessment is the safe choice. Do not wait until the next morning or your next scheduled appointment.
Will diarrhoea happen with every cycle of chemotherapy?
Not necessarily, and the pattern varies from person to person and from drug to drug. Some people have it after one cycle and not after others. Some regimens are more likely to cause it than others. If you had diarrhoea with your first cycle, tell your team before the next one, so they can decide whether any adjustment to your supportive care plan would help.
When will the diarrhoea get better?
For most people, diarrhoea related to chemotherapy eases as the gut lining recovers between cycles. How long that takes depends on the drug you are receiving and your individual response. Persistent diarrhoea that does not settle between cycles is worth reporting at your next appointment, even if it was not severe enough to warrant a same-day call. Your team can review your supportive care plan and adjust it if needed.
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