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Alternating constipation and diarrhoea: what it usually means | CION Cancer Clinics
Most people whose bowels swing between constipation and loose motions do not have cancer. Irritable bowel syndrome, overflow from a blocked-up bowel, a new tablet or a change in diet explain the pattern far more often. A change that is new, lasts for weeks, or comes with bleeding, tiredness or weight loss is the one that needs a doctor. This page explains how to tell the difference 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.
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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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On this page
The short answer
Is it normal to swing between constipation and loose motions?
Most people whose bowels swing between constipation and diarrhoea do not have cancer. The commonest reason by far is irritable bowel syndrome, a disorder of how the gut moves that is uncomfortable but not dangerous. A change in diet, a course of antibiotics, a new tablet, stress and overflow from a blocked-up bowel all produce the same pattern. Colon cancer is on the list, but it sits near the bottom of it.
Why the pattern happens at all
The colon's job is to pull water out of what the small bowel sends it. If things move too slowly, stool dries out and you are constipated. If they move too fast, water stays in and you get loose motions. Anything that makes the gut's rhythm unreliable, from nerves to an infection to a narrowing, can tip it one way and then the other within the same week.
What makes a doctor look harder
Age, and whether the pattern is new. A person who has had an unpredictable gut since college, with no bleeding and no weight loss, is almost always describing a functional problem. A person past middle age whose bowels were regular for decades and have changed in the last few months is describing something that needs a proper look, even if the answer turns out to be harmless. That is the whole distinction, and it is why the change matters more than the pattern itself.
A new change in bowel habit that lasts is a reason to be examined. It is not a diagnosis.The usual reasons
What usually causes alternating constipation and diarrhoea?
Four explanations account for most people who search this phrase. Cancer is not one of the four.
Irritable bowel syndrome
The gut is structurally normal but its movement is disordered. Cramping that eases after passing stool, bloating, and bowels that alternate without warning are the classic picture. It usually starts young and comes and goes for years.
Points towards IBS
- Worse with stress, exams or travel
- No blood, no weight loss, no night symptoms
- Pain relieved by a bowel motion
Overflow from constipation
Hard stool packs the lower bowel and liquid stool from above leaks around it. The person thinks they have diarrhoea when the real problem is severe constipation. Common in older people, in those on strong painkillers and in anyone who has cut back on water and fibre.
Treating the "diarrhoea" with anti-motility tablets makes this worse.Diet, infection and medicines
A bout of food poisoning can leave the gut unsettled for weeks. Antibiotics, iron tablets, diabetes drugs, antacids and some blood pressure tablets each push the bowel one way. A sudden switch, such as moving from millets to polished rice or starting a protein powder, does the same.
Bowel disease, including cancer
Ulcerative colitis, Crohn's disease, diverticular disease and a growth in the colon can all narrow or inflame the bowel so that stool passes irregularly. These are the reasons a doctor wants to rule out, and they are more likely when the change is new, persistent and comes with bleeding, tiredness or weight loss.
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When this needs a doctor this week, and when it needs one today
See a doctor this week if the alternating pattern is new, has lasted more than a few weeks, or comes with blood in the stool, unexplained tiredness, weight you did not try to lose, or you are past middle age. Go to an emergency department today if you cannot pass stool or wind at all, your abdomen is swollen and painful, you are vomiting, or you are passing large amounts of blood.
At the appointment
What will the doctor actually do about it?
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Ask about the pattern, not just the symptom
When it started, what your bowels were like before, what the stool looks like, whether there is blood or mucus, and whether anyone in the family has had bowel cancer or polyps. A symptom diary makes this part much quicker.
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Examine the abdomen and, usually, the back passage
A gloved finger examination takes a minute and is embarrassing but not painful. It finds hard stool, piles and low growths. A normal examination is reassuring but does not see the colon itself.
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Blood tests and a stool test
A full blood count looks for anaemia, meaning a low haemoglobin, which can be the only clue to a bleeding growth. A stool test for hidden blood, such as a FIT test, helps decide who needs the colon looked at. A negative stool test does not rule cancer out when symptoms are present.
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A colonoscopy if anything points that way
A camera passed through the whole colon, under sedation, at a diagnostic partner. It is the only test that both sees a growth and takes a biopsy, a small sample of tissue, from it. CION arranges it and reviews the result with you.
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Commonly believed
Four things people assume about an unreliable bowel
"It is just my digestion. Everyone in our family has a weak stomach."
Often true, and if it has been this way for years it probably is. But a family where several people have "weak stomachs" is also a family where nobody has had a colonoscopy. A long-standing pattern that suddenly changes in character deserves the same attention as a brand-new one.
"If it were cancer there would be blood."
Not always. Growths on the right side of the colon often bleed so slowly that the stool looks normal and the only sign is tiredness from anaemia. Some people present with a change in bowel habit and no visible bleeding at all.
"I am too young for this to be anything serious."
Most younger adults with this pattern have IBS, and that is the right first assumption. Colon cancer in people under fifty is rising, though, and is often found late precisely because everyone assumed it could not be. Persistent new symptoms deserve a proper look at any age.
"An ultrasound was normal, so the colon is fine."
An abdominal ultrasound looks at the liver, kidneys and gall bladder. It cannot see inside the colon and a normal report says nothing about it. Only a colonoscopy, or in some cases a CT scan of the colon, does that.
Being straight with you
What this page cannot tell you
It cannot tell you whether your bowel is healthy. No description of a symptom can do that, because the benign causes and the serious ones produce exactly the same pattern from the outside. What it can do is help you decide how urgently to be seen, and what to say when you get there.
Who this page is not written for
If you already have a diagnosis of ulcerative colitis or Crohn's disease, a change in your bowels is a question for your gastroenterologist, because flares and the cancer risk in long-standing colitis follow their own rules. If your symptoms are mainly in the back passage, with pain on passing stool and a feeling of not emptying fully, the rectal cancer hub is closer to your question than this one.
Who can answer it
A family physician or a gastroenterologist, after examining you and usually after a blood test and a stool test. If a colonoscopy is advised, CION arranges it through a diagnostic partner and the report comes back to a CION oncologist only if something is found that needs one. Most people who go through this process are told their colon is clear.
If you are unsure whether what you are describing is worth an appointment, call the helpline and describe it. Someone will tell you honestly.Questions we are asked
Common questions about alternating bowel habit
Is alternating constipation and diarrhoea a sign of colon cancer?
It can be, but in most people it is not. IBS, overflow constipation, diet changes and medicines are far more common explanations. A doctor becomes more concerned when the pattern is new in someone past middle age, lasts for weeks, or comes with bleeding, anaemia or weight loss.
How long should I wait before seeing someone?
If the pattern has lasted more than a few weeks and is not clearly explained by a recent infection, a new tablet or a diet change, book an appointment now rather than waiting for it to settle. Bleeding, tiredness or weight loss alongside it means do not wait at all.
Can IBS turn into colon cancer?
No. IBS does not damage the bowel lining and does not raise the risk of cancer. The difficulty is that cancer can arrive in someone who also has IBS and be mistaken for a flare, which is why a change in a long-standing IBS pattern should be checked.
What is overflow diarrhoea?
When hard stool blocks the lower bowel, liquid stool from higher up leaks around it and comes out as watery motions. The person feels they have diarrhoea but the real problem is severe constipation. It is common in older people and in anyone on strong painkillers.
Which side of the colon causes a change in bowel habit?
Usually the left side, where stool is already formed and the bowel is narrower, so a growth there changes how stool passes. Right-sided growths more often cause tiredness and anaemia rather than a change in habit. Either can occur with alternating bowels.
Will a stool test tell me whether it is cancer?
No. A FIT or similar stool test looks for hidden blood and helps decide who needs a colonoscopy. A positive result means the colon should be looked at, not that cancer is present. A negative result does not rule cancer out when you have ongoing symptoms.
Should I change my diet first and see if it settles?
More water, regular meals and steady fibre from vegetables, fruit and whole grains help most functional bowel problems and are worth doing. Do not let a diet trial delay an appointment if you are past middle age, or if the change came with bleeding or weight loss.
Which doctor should I see in Hyderabad?
A family physician or general physician is the right first step, and a gastroenterologist if a colonoscopy is being considered. You do not need an oncologist unless something is found. CION can arrange the colonoscopy through a diagnostic partner and review the report if it ever needs an oncologist's eye.
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Sources
- NHS — Symptoms of bowel cancer
- NHS — Irritable bowel syndrome (IBS)
- American Cancer Society — Colorectal Cancer Signs and Symptoms
- NHS — Constipation
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 mattersThe wider question this page sits insideNew constipation in an adultIf the constipated phase is the one that worries youPersistent diarrhoea and colon cancerIf the loose phase is the one that worries youWhat the doctor will ask and examineSo the first appointment holds no surprises
Talk to us
Not sure whether your bowel change is worth an appointment?
Describe what has changed and for how long, and we will tell you honestly whether it needs a check and arrange a colonoscopy through a diagnostic partner if it does. One helpline serves every CION centre.