Change in Bowel Habits: When Do You Need a Colonoscopy?

A change in bowel habits is common. It can happen after a stomach bug, a stressful period, a change in diet, travel, a new medication or simply a disruption to your usual routine.

Most changes are not caused by bowel cancer. But a persistent or unexplained change deserves attention—particularly when it is new for you, lasts more than a few weeks, or comes with symptoms such as bleeding, weight loss, anaemia or abdominal pain.

A colonoscopy is not needed for every episode of constipation or diarrhoea. It may, however, be the right test when symptoms suggest that the bowel needs a closer look.

Quick answer: what counts as a genuine change in bowel habit?

Your “normal” bowel habit is personal. Some people open their bowels every day; others go every second day. Some people naturally have softer stools, while others tend towards constipation.

A genuine change is a noticeable difference from your usual pattern that persists or keeps returning. This may include:

• new constipation
• new or persistent diarrhoea
• alternating constipation and diarrhoea
• opening your bowels much more often than usual
• urgency or difficulty holding on
• a feeling that the bowel has not fully emptied
• a change in stool appearance, including narrower stools
• ongoing mucus, bloating or abdominal discomfort

A short-lived change after gastroenteritis, antibiotics, travel or a dietary change is often less concerning. If symptoms have not settled within around three weeks, are getting worse, or occur with warning signs, speak with your GP.

When is a change in bowel habit more concerning?

The pattern matters. So do your age, medical history, medications and family history.

New constipation

Constipation means stools are hard, difficult to pass or less frequent than is normal for you. It is often related to diet, dehydration, reduced activity, certain medicines or a change in routine.

New constipation can still need investigation when it is persistent, especially if it comes with abdominal pain, bloating, vomiting, rectal bleeding, weight loss or iron-deficiency anaemia.

New or persistent diarrhoea

Diarrhoea is common after a viral illness, food poisoning or antibiotics. It can also occur with food intolerance, coeliac disease, inflammatory bowel disease, medication side effects or irritable bowel syndrome.

Diarrhoea that continues beyond a few weeks, wakes you from sleep, contains blood, causes weight loss or occurs with fever or significant pain should be assessed. Your doctor may arrange blood tests, stool tests or a referral for colonoscopy depending on the situation.

Alternating constipation and diarrhoea

Some people experience swings between constipation and diarrhoea. This can occur with IBS and functional gut symptoms, but IBS should not simply be assumed when symptoms are new, changing or accompanied by red flags.

A careful assessment helps distinguish a functional bowel condition from inflammation, infection, medication effects or a structural problem in the bowel.

Going more often, urgency or incomplete emptying

Needing to open your bowels more often than usual, rushing to the toilet, or feeling you have not completely emptied can have many causes. These include infection, inflammation, haemorrhoids, changes in diet and bowel sensitivity.

When these symptoms are persistent or occur with bleeding, pain, mucus, anaemia or weight loss, your GP may recommend further investigation.

Narrower stools

Thin or narrower stools can be caused by a temporary change in bowel function, loose stool consistency or muscle spasm. On their own, they do not diagnose bowel cancer.

However, a persistent change in stool calibre—particularly alongside rectal bleeding, abdominal pain, weight loss, fatigue or a family history of bowel cancer—should be discussed with a doctor.

Red flags: when to seek medical advice promptly

Most changes in bowel habit have a benign explanation. The following symptoms make it more important to arrange a timely GP review:

• blood in the stool, on the toilet paper or in the toilet bowl
• black or tar-like stools
• unexplained iron-deficiency anaemia
• unintentional weight loss
• persistent abdominal pain, bloating or a lump
• symptoms that wake you at night
• fever, persistent vomiting or feeling significantly unwell
• a family history of bowel cancer, advanced bowel polyps, inflammatory bowel disease or certain inherited cancer syndromes
• a new and ongoing bowel change later in life
• a positive bowel screening test

If you have severe abdominal pain, heavy bleeding, fainting, fever with worsening pain, or are unable to keep fluids down, seek urgent medical care rather than waiting for a routine appointment.

What else can cause a change in bowel habits?

A change in bowel habit does not automatically mean cancer. In fact, common causes include:

Infection

Gastroenteritis, food poisoning and some bacterial infections can cause diarrhoea, cramping and urgency. Symptoms usually improve, but prolonged diarrhoea may need stool testing.

Medications and supplements

Iron tablets commonly cause constipation and darker stools. Antibiotics can cause diarrhoea. Medicines for pain, diabetes, weight loss, blood pressure and mental health can also affect bowel function.

Bring an up-to-date medication list to your appointment, including over-the-counter products and supplements.

Diet and lifestyle changes

Not drinking enough fluid, reducing physical activity, increasing alcohol intake, changing fibre intake or eating differently while travelling can all change bowel habits.

IBS and functional gut conditions

IBS can cause pain, bloating, constipation, diarrhoea or alternating bowel habits. It is common and does not turn into bowel cancer. However, a diagnosis of IBS should be made thoughtfully, especially when symptoms are new or there are warning signs. Read more about IBS and functional gut symptoms.

Diverticular disease or inflammation

Diverticular disease, inflammatory bowel disease and microscopic colitis can all cause ongoing bowel symptoms. Colonoscopy may help identify inflammation, obtain biopsies or rule out other causes where appropriate.

Haemorrhoids and anorectal conditions

Haemorrhoids can cause bright-red bleeding, itching, discomfort or a lump near the anus. They do not explain every episode of bleeding, however. New or persistent bleeding should not automatically be assumed to be haemorrhoids. See our guide to haemorrhoids and rectal bleeding.

FIT versus colonoscopy: are they the same thing?

No. A FIT, sometimes called an FOBT or bowel screening test, looks for tiny amounts of blood in a stool sample. It is an important screening test for people without symptoms.

Colonoscopy is different. It allows a specialist to inspect the inside of the large bowel directly, take biopsies and remove many polyps during the same procedure.

If you have persistent symptoms—such as a continuing change in bowel habit, rectal bleeding, iron-deficiency anaemia, unexplained weight loss or abdominal pain—a screening test is not a substitute for medical assessment. Your GP will decide whether a FIT, blood tests, stool tests, imaging, colonoscopy or another pathway is most appropriate.

For more information about the National Bowel Cancer Screening Program and positive FIT results, see our bowel cancer screening guide.

What will your GP or specialist check first?

Assessment starts with your story. Your doctor may ask:

• When did the change begin?
• Is it getting better, worse or staying the same?
• Have you noticed blood, mucus, pain, bloating or urgency?
• Have you lost weight without trying?
• Are you tired, dizzy or short of breath?
• Have you recently travelled, taken antibiotics or changed medication?
• Is there a family history of bowel cancer, polyps, coeliac disease or inflammatory bowel disease?

You may have an examination and tests such as a full blood count, iron studies, inflammatory markers, coeliac blood tests or stool testing. These results, along with your symptoms and history, help determine whether colonoscopy is appropriate.

What happens if a colonoscopy is recommended?

A colonoscopy is a day procedure that examines the large bowel with a thin, flexible camera. It can help identify inflammation, polyps, bleeding sources, diverticular disease and other bowel conditions.

Before the procedure, the bowel must be cleaned with a prescribed preparation. Good preparation is important because it allows the bowel lining to be examined properly. Our colonoscopy preparation guide explains the usual process, but always follow the individual instructions supplied for your procedure.

Most patients have sedation and go home the same day with a responsible adult. If polyps are found, they can often be removed during the procedure and sent to the laboratory for testing. Dr Goutham will explain the initial findings after the procedure, while biopsy and pathology results may take longer.

Booking a colonoscopy in Brisbane

If your GP has recommended a colonoscopy, or you are concerned about a persistent change in bowel habit, Colonoscopy Brisbane can guide you through the next steps.

Dr Goutham Sivasuthan provides assessment, colonoscopy and follow-up across Brisbane, Moreton Bay, Redland and Logan. A GP referral is usually helpful because it supports Medicare rebates for specialist consultations, but you can also contact the rooms directly to discuss your options.

For insured patients, no-gap options may be available for the surgeon’s fee. Fixed-price options are also available for eligible self-funded patients. See our colonoscopy cost and no-gap guide for current information.

The bottom line

A change in bowel habits is often caused by something manageable and non-serious. But if the change is new, persistent, unexplained or accompanied by bleeding, weight loss, anaemia, pain or a family history of bowel cancer, do not ignore it.

The right next step is usually a GP review. If colonoscopy is recommended, it can provide answers, identify treatable conditions and remove many bowel polyps before they become a bigger problem.

Frequently asked questions

How long should I wait before seeing a GP about a change in bowel habits?

A short-lived change after a stomach bug, antibiotics, travel or a change in diet is often not concerning. If your symptoms haven’t settled within around three weeks, are getting worse, or occur with red flags such as bleeding, weight loss or abdominal pain, book a GP review.

Does a change in bowel habits always mean bowel cancer?

No. Most changes in bowel habit are caused by something manageable, such as infection, medication, diet, lifestyle changes or a functional condition like IBS. A persistent or unexplained change, especially with red-flag symptoms, still deserves assessment.

What’s the difference between a FIT test and a colonoscopy?

A FIT (bowel screening) test checks a stool sample for tiny amounts of blood and is used for people without symptoms. Colonoscopy directly examines the large bowel, can take biopsies and remove polyps, and is used when you have persistent symptoms rather than as a screening substitute.

Can IBS explain alternating constipation and diarrhoea?

IBS is a common cause of alternating bowel habits, but a diagnosis of IBS should be made carefully, particularly when symptoms are new, changing, or occur with warning signs such as bleeding, weight loss or a family history of bowel cancer.

Do I need a GP referral for a colonoscopy?

A GP referral is usually helpful, as it supports Medicare rebates for specialist consultations. You can also contact Colonoscopy Brisbane directly to discuss your options.

Reviewed by Dr Goutham Sivasuthan, Specialist Surgeon and GESA-accredited endoscopist — July 2026.

This article provides general information and is not a substitute for personalised medical advice. Please discuss your symptoms and bowel-habit changes with your GP or a specialist.