Colonoscopy: when you need one, preparation, the procedure and risks

What a colonoscopy is

During a colonoscopy, a colonoscope – a flexible tube about the width of a finger, with a light and a video camera at the tip – is passed through the anus. A gastroenterologist guides it through the rectum and the whole colon to the point where it joins the small bowel. The image is shown on a high-definition screen, and instruments can be passed through a channel in the scope to take biopsies and remove polyps.

The test usually takes 20 to 45 minutes. Modern equipment uses high-definition imaging and special light settings that help reveal small and flat polyps, and artificial intelligence systems are increasingly used to help the doctor spot them.

When a colonoscopy is recommended

• Screening – in people without symptoms, usually from about the age of 50, or after a positive faecal immunochemical test (FIT). Read more in early detection of cancer.
• Symptoms – blood in the stools, a lasting change in bowel habit, unexplained anaemia, weight loss or persistent abdominal pain.
• Higher risk – a close relative with bowel cancer or advanced polyps, or an inherited syndrome such as Lynch syndrome or familial polyposis.
• Surveillance – after polyps have been removed, after surgery for colorectal cancer or in long-standing inflammatory bowel disease such as ulcerative colitis or Crohn's disease.

If you take anticoagulants or have serious heart or lung conditions, the test may be planned in a hospital setting where longer monitoring is available. Ask in advance whether sedation and laboratory analysis of any samples are included.

Preparing for a colonoscopy

The bowel needs to be completely clean; otherwise small polyps can be missed and the test may have to be repeated. Your endoscopy team will give you specific instructions, but preparation usually involves:

• A low-fibre diet for 2–3 days – avoiding seeds, nuts, fruit skins, wholegrain foods and raw vegetables; white bread, rice, chicken, fish and eggs are usually fine.
• Clear fluids only the day before – water, clear broth, tea and clear juice without pulp.
• Bowel preparation solution – usually taken as a split dose, part the evening before and part a few hours before the test, which gives the best result. The aim is for what you pass to become clear and yellowish.
• Medicines – tell the team in advance if you take blood thinners, diabetes medicines or iron tablets. Do not stop any medicine on your own; the doctor will tell you which ones to pause and for how long.

If you will be sedated, arrange for someone to collect you afterwards, as you must not drive for the rest of the day.

What happens during the test

You lie on your left side. Most centres offer intravenous sedation – a light anaesthetic that makes you relaxed or drowsy, and many people do not remember the procedure. The test can be done without sedation, but you may then feel uncomfortable cramps.

Air or carbon dioxide is gently passed into the bowel to open it up. Any polyps found are usually removed straight away with a small wire loop or forceps (polypectomy) and sent to the laboratory. Biopsies are also taken from any suspicious areas.

Afterwards you rest briefly while the sedation wears off. Bloating and wind are normal for a few hours, and you can usually eat straight away unless told otherwise. You will be told what was seen on the day; biopsy results usually take one to two weeks.

Risks and complications

Colonoscopy is a safe test, but like any procedure it carries some risks:

Seek urgent medical help if, after the test, you develop severe or worsening abdominal pain, a fever, heavy bleeding or black stools, or feel faint or dizzy.

Alternatives to colonoscopy

• Faecal immunochemical test (FIT) – a simple, non-invasive screening test; a positive result is followed by colonoscopy.
• CT colonography (virtual colonoscopy) – a scan-based test (see MRI and CT scans); it also needs bowel preparation but cannot remove polyps or take biopsies.
• Capsule colonoscopy – swallowing a capsule containing a camera; used in limited situations.
• Flexible sigmoidoscopy – examines only the lower part of the bowel.

Colonoscopy remains the gold standard because it combines diagnosis and treatment in a single test.

How often to repeat it, and when to see a doctor

If the result is normal and your risk is average, colonoscopy is usually repeated after ten years. After polyps are removed, the interval depends on their number, size and type – most often between three and ten years. Intervals are shorter for inherited syndromes and inflammatory bowel disease.

Talk to your GP about a referral if:

• you are 50 or over and have never been screened;
• you have blood in your stools, a lasting change in bowel habit or unexplained anaemia;
• a close relative has had bowel cancer – testing is then often advised from 40, or ten years before the age at which your relative was diagnosed.

If a tumour is found, the next steps are explained in our guide to cancer treatment.

Colonoscopy abroad and in Türkiye

Some patients from Bulgaria and other European countries have a colonoscopy, including under sedation, at large accredited hospital groups in Istanbul, often as part of a wider health check-up. Check that the test is performed by an experienced gastroenterologist, that an anaesthetist supervises sedation, and that you will receive a written report with images and the biopsy results – and how you will be told if these arrive after you return home. Organisers such as New City Clinic coordinate the hospital, doctor, hotel, transfers and interpreter. If a significant finding is made, a second opinion is a sensible step.