Bowel cancer screening and colonoscopy
A plain-language guide for patients
The short version
- Bowel cancer (cancer of the large bowel — the colon and rectum) is one of Australia's most common cancers, but it is also one of the most preventable. Found early, more than 9 in 10 cases can be treated successfully.
- Most bowel cancers grow slowly from a small growth called a polyp, over about 10 to 15 years. Screening looks for cancer, or for the polyps that lead to it, before they cause harm.
- In Australia, screening starts with a free home test every 2 years from age 45. A colonoscopy looks directly inside the bowel — after a positive home test, or as the main test for people at higher risk.
- A colonoscopy is unusual: it can find a polyp and remove it in the same procedure.
- Screening works — but the benefit depends on actually having the test, and on it being done well.
1 Why screen for bowel cancer?
Bowel cancer often causes no symptoms until it is advanced. Screening aims to find it — or the polyps that lead to it — before that happens.
Around 15,000 Australians are diagnosed with bowel cancer each year, and it remains one of the leading causes of cancer death. The encouraging part is that it is largely preventable. Almost all bowel cancers begin as a polyp, a small growth on the bowel lining, which grows slowly over many years before it can turn into cancer.
This slow timeline is what makes screening so effective. If a polyp is found and removed, the cancer it might have become is prevented. And if a cancer is found early, before symptoms appear, it is far more likely to be curable — more than 90% of early bowel cancers can be treated successfully.
2 How screening works in Australia
There are two tests, used for different people and different purposes.
The home test (iFOBT)
A free, simple test you do at home. It is mailed to everyone aged 50 to 74 every 2 years, and people aged 45 to 49 can request their first kit. It checks your stool for tiny, invisible traces of blood. It does not diagnose cancer — a positive result means a colonoscopy is recommended to find the cause.
Colonoscopy
A camera test that looks directly at the whole lining of the bowel, usually under sedation. It is used after a positive home test, and as the main screening test for people at higher risk. Unlike the home test, it can remove polyps during the same procedure.
The best test is the one that gets done
Returning your free home-test kit every 2 years from age 45 is one of the simplest and most effective things you can do for your health. If your kit hasn't arrived, you can request one or ask your GP.
3 What a colonoscopy does, and what the evidence shows
A colonoscopy both looks for cancer and helps prevent it — by finding and removing polyps before they can cause harm.
Because the doctor can remove polyps during the procedure, a colonoscopy is both a test and a treatment in one. How much this helps in practice was studied carefully in a major clinical trial.
The NordICC trial, published in 2022, is the first large randomised trial of colonoscopy screening. It invited more than 84,000 healthy people aged 55 to 64 in Poland, Norway and Sweden either to a single screening colonoscopy or to usual care, and compared them 10 years later. Importantly, only about 4 in 10 of the people invited (42%) actually had the colonoscopy. The results are best understood as two different numbers:
Counting everyone who was invited — including the roughly 6 in 10 who never attended — there were 18% fewer bowel cancers over 10 years. Deaths from bowel cancer were also lower, but the difference was small and not yet statistically certain at the 10-year mark.
Counting only the people who actually had the colonoscopy, the benefit was substantially larger: about 31% fewer bowel cancers and about 50% fewer deaths from bowel cancer.
What this means for you
The clearest message from NordICC is simple: a colonoscopy can only protect you if you actually have it. The trial also reported relatively early — polyps take years to become cancer, so the full benefit is expected to grow by the planned 15-year results and when more people take part. Taken together with earlier screening trials, the evidence supports colonoscopy screening, provided it is done, and done well.
4 The benefits, and the risks
Colonoscopy is a safe and very commonly performed procedure, but, like any test, it is not without risk.
Benefits
Finds and removes polyps in a single procedure, which can prevent bowel cancer; finds cancer early, when it is most treatable; and, for most people, provides reassurance.
Risks
Serious problems are uncommon. In the NordICC trial there were no perforations (a tear in the bowel wall), major bleeding after polyp removal occurred in about 1 in 800 people, and there were no deaths in the 30 days after the procedure. In general practice, perforation occurs in roughly 1 in 1,000 colonoscopies. There are also the minor effects of the bowel preparation and the small risks of sedation.
No test is perfect
Some polyps — especially flat or pale ones — can be hard to see, so a small number are missed even during a careful colonoscopy. This is why a thorough technique and excellent bowel preparation matter so much, and why screening is repeated over time rather than relied on as a one-off.
5 Preparing for your colonoscopy, and what to expect
Bowel preparation is part of the test
The bowel must be completely clean for polyps to be seen and removed. Following the preparation instructions carefully is one of the most useful things you can do to make your colonoscopy effective.
On the day, the procedure itself usually takes about 20 to 30 minutes and is done under sedation, so most people remember little or nothing of it. You will need someone to take you home afterwards and should not drive for the rest of the day. Your specialist will explain what was found; any polyps removed are sent to the laboratory, and results with a clear follow-up plan are provided to you and your GP.
6 Family history and higher risk
Some people need colonoscopy-based screening rather than the home test — usually earlier, and more often.
Your risk is higher than average if you have a first-degree relative (parent, brother or sister, or child) diagnosed with bowel cancer or advanced polyps, particularly at a younger age; a personal history of polyps, bowel cancer, or inflammatory bowel disease; or a known inherited condition such as Lynch syndrome or familial adenomatous polyposis.
If any of these apply to you
Talk to your doctor. You may need to begin screening before age 45, have colonoscopy rather than the home test, and repeat it at set intervals. Telling your close relatives about bowel cancer or polyps in the family lets them arrange screening at the right time too.
7 What you can do
A few simple steps make a real difference.
Take part in screening
Return your free home-test kit every 2 years from age 45, and attend any colonoscopy you are advised to have, on time.
Do not ignore symptoms
See your doctor about bleeding, a lasting change in bowel habit, unexplained weight loss, or ongoing tummy pain — even if you have screened recently (see the box below).
Know your family history
Tell your doctor about bowel cancer or polyps in close relatives, so your screening can be tailored to you.
Keep bowel-healthy habits
Plenty of fibre, vegetables and wholegrains; less red and processed meat and alcohol; regular activity; a healthy weight; and not smoking.