Medical Oncologist
CION Cancer Clinics
Diarrhoea that will not settle: could it be colon cancer? | CION Cancer Clinics
Diarrhoea that lasts for weeks is rarely colon cancer. Infections, irritable bowel syndrome, medicines and an inflamed gut explain most cases. It does need a doctor when it is new for you and will not settle, or when it comes with blood, weight loss or night-time motions. This page explains the common causes, the warning signs and what the doctor will do. At CION Cancer Clinics, our specialists help you judge which bowel symptoms need tests and plan screening that fits your risk.
NG
Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed October 2026, next review due October 2027
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The short answer
Can diarrhoea that will not settle be a sign of colon cancer?
It can be, but it rarely is. Most loose motions that drag on for weeks come from an infection, a medicine, irritable bowel syndrome or an inflamed gut. Colon cancer sits low on that list, and it matters mainly when the diarrhoea is new for you, will not go away and arrives with other warning signs.
Why a tumour can cause loose stools
A growth in the colon can narrow the passage. Hard stool backs up behind it, and only liquid squeezes past, so the person sees frequent watery motions. Some tumours also make extra mucus. This is why bowel cancer more often shows as a lasting change in habit, sometimes swinging between diarrhoea and constipation, than as one bad week.
What makes doctors look more closely
A new pattern that has lasted several weeks, blood mixed into the stool, waking at night to pass motion, weight loss you did not plan, or a low haemoglobin on a blood test. Age and a family history of bowel cancer add weight. Any one of these is reason to see a doctor, which is a different thing from being told you have cancer. For the wider picture, read our page on a change in bowel habit.
Diarrhoea alone, with none of these signs, is very unlikely to be cancer.What it usually is
What else causes diarrhoea that lasts for weeks?
These explain far more lasting diarrhoea than cancer does. Several are easy to test for and treat.
Infections and parasites
A stomach bug usually clears within a week. Some, such as giardia and amoebic infection, settle in and keep the motions loose for weeks until they are found on a stool test and treated.
Repeated courses of antibiotics from the pharmacy can hide the pattern without fixing it.Irritable bowel syndrome
A common, long-running problem with urgency, cramps and bloating that often eases after a motion. It is real and treatable, though it does not damage the bowel.
Usually looks like
- Years of ups and downs, often worse with stress
- No blood and no weight loss
- Rarely wakes you at night
Medicines and food
Metformin for diabetes, some antacids, antibiotics and iron syrups can all loosen stools. So can milk in people who no longer digest it well, and a lot of tea, coffee or alcohol.
An inflamed or sensitive gut
Ulcerative colitis, Crohn's disease, coeliac disease, an overactive thyroid and, in India, tuberculosis of the gut can all cause weeks of diarrhoea. They need proper tests, and most are very treatable once named.
Not sure whether this applies to you?
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When to see a doctor this week, and when to go today
See a doctor this week if loose motions have lasted more than a few weeks, or come with blood, weight loss, night-time motions or tiredness. Go to an emergency department today if you are passing a lot of blood or black stools, cannot keep fluids down, feel dizzy or confused, pass very little urine, or have severe belly pain with a swollen tummy. Read more on when bowel symptoms are an emergency.
At the appointment
What will the doctor do to find the cause?
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Questions about the pattern
How long it has lasted, how often you go, whether it wakes you at night, any blood or mucus, weight change, recent travel, new medicines and family history. A short diary of your motions makes this much easier.
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An examination
The doctor feels your belly and may do a gentle examination of the back passage with a gloved finger. It takes a minute, and it checks for bleeding, a lump or a cause near the anus.
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Blood and stool tests
A blood count to look for anaemia, which means a low haemoglobin, plus thyroid and coeliac checks. Stool tests look for infection and parasites, and a stool marker of gut inflammation helps separate irritable bowel from colitis.
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A colonoscopy if the signs point that way
If there is blood, anaemia, weight loss or a raised inflammation marker, a camera test of the whole colon is the usual next step. CION arranges it through accredited partner centres. See what a diagnostic colonoscopy involves.
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A plan based on what is found
Most people leave with a named benign cause and a treatment. If a growth is found, a small sample is taken and the CION tumour board plans the next steps with you.
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Commonly believed
Four things people say about loose motions, and what is true
"Colon cancer causes constipation, not diarrhoea."
It can cause either, or both in turn. What matters is a lasting change from what is normal for you, whichever direction it goes.
"There is no blood, so it cannot be anything serious."
Blood is a warning sign, but its absence does not settle anything. Some tumours bleed slowly and invisibly, and only show up as tiredness and a low haemoglobin.
"It is just the water, or outside food."
For a few days, that is often right. When it has gone on for weeks despite home food, boiled water and ORS, the cause needs to be found rather than guessed.
"My stool test was normal, so the colon is fine."
A normal infection test only rules out infection. Even a negative stool blood test does not rule out cancer when symptoms are present. Persistent symptoms still need a doctor's review.
Being straight with you
What this page cannot tell you
It cannot tell you why your own diarrhoea has lasted. That needs a doctor who has heard the whole story, examined you and seen the results of a few simple tests. No list of causes, however long, can stand in for that.
Who this page does not apply to
If you are already on chemotherapy, targeted therapy or immunotherapy, diarrhoea is usually a treatment effect and has its own rules about when to call. Read diarrhoea during chemotherapy and call your treating team the same day. Young children with diarrhoea need their paediatrician, because they dry out much faster than adults.
When the problem may be lower down
If the main trouble is a constant urge to go, with little coming out, or bleeding close to the anus, the cause may sit in the rectum or anus. Those cancers are separate hubs, at rectal cancer and anal cancer, and a doctor can tell you which one applies.
If you are unsure whether your symptoms need a test, call the helpline and describe them. Someone will tell you honestly.Questions we are asked
Common questions about long-lasting diarrhoea
How long is too long for diarrhoea to last?
Most stomach upsets settle within a week. If loose motions go on for more than a few weeks, or keep coming back, see a doctor even if you feel otherwise well. Go sooner if there is blood, weight loss, a fever or signs of drying out.
Does colon cancer diarrhoea look different?
Not in a way you can judge at home. It may be frequent and watery, sometimes with mucus or dark blood, and it may alternate with constipation. Many harmless conditions look exactly the same, which is why the pattern and the tests matter more than the appearance.
I am in my thirties. Should I still worry?
Colon cancer is uncommon at your age, and infection, irritable bowel and colitis are far more likely. It is not impossible, though. If symptoms persist, carry warning signs or there is bowel cancer in a parent or sibling, ask for proper tests rather than accepting more of the same tablets.
Can stress alone cause weeks of loose motions?
Stress can trigger and worsen irritable bowel syndrome, which does cause long spells of diarrhoea. It should not cause bleeding, weight loss or night-time motions. If any of those are present, do not put it down to stress until a doctor has checked.
Will a stool test show colon cancer?
A stool test for hidden blood can help decide who needs a colonoscopy, but it does not diagnose cancer. A positive result leads to a colonoscopy. A negative result does not rule cancer out if you still have symptoms, so the doctor may arrange one anyway.
Which doctor should I see first?
Your family doctor or a general physician is a sensible start. A gastroenterologist sees bowel symptoms every day and can arrange a colonoscopy if needed. Our page on which doctor to see explains when each one fits.
Should I stop the antibiotics my pharmacist gave me?
Finish or stop any course only on a doctor's advice. Repeated courses taken without a diagnosis can upset the gut further and blur the picture. Tell the doctor exactly what you took and for how long, ideally by bringing the strips with you.
How can CION help with this?
If you or a parent has unexplained diarrhoea with warning signs, CION can review the history and reports, arrange a colonoscopy through a partner centre, and explain the result in plain language. Call the helpline and describe the symptoms. Someone will guide you to the right next step.
Your Specialists
Meet CION's colon cancer specialists. Bring your colonoscopy, scan or biopsy report.
Our surgical and medical oncologists plan colon cancer care together: which operation, whether chemotherapy is needed, and the follow-up that comes after.
Medical Oncologist
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Medical Oncologist
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
Medical Oncologist
Medical Oncologist
Surgical Oncologist
Dr. Muralidhar Muddusetty
MBBS (AIIMS), MS (Surgery) (AIIMS), DNB (Surgical Oncology), MRCS (Edinburgh)
Surgical Oncologist
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Sources
- NHS — Diarrhoea and vomiting
- NHS — Symptoms of bowel cancer
- NHS — Irritable bowel syndrome (IBS)
- NICE — Suspected cancer: recognition and referral
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
Change in bowel habit: when it mattersHow doctors judge any lasting change, not just diarrhoeaAlternating constipation and diarrhoeaIf your motions swing between the twoWhat the doctor will ask and examinePrepare for the appointment before you goWhen bowel symptoms are an emergencyThe signs that mean hospital today
Talk to us
Loose motions for weeks and no clear answer?
Tell us what has been happening and what tests you have had. We will help you decide whether a colonoscopy is needed and arrange it if it is. One helpline serves every CION centre.