A colonoscopy is performed for both diagnostic and therapeutic reasons, and commonly done with sedation.

Common reasons for the test:

1. to diagnose lower gastrointestinal (GI) tract disease that causes abdominal pain and/or bleeding from back passage (rectum)
2. to diagnose a tumour in the presence of ‘alarm’ features including a change in bowel habit consisting of diarrhoea (increased bowel frequency or loose stool or both), weight loss or iron deficiency anaemia
3. family history of colon cancer
4. follow up test for abnormal barium enema or CT colonography
5. follow up examination for past history of colon polyp(s)


The alternative tests to examine the colon include a barium enema or CT colonography examination. These have the disadvantage that tissue samples (biopsies) cannot be taken for analysis. If a polyp (a protrusion or growth on the bowel lining) is diagnosed, a colonoscopy would be needed for polyp removal. They are reasonable alternatives in the case of a failed colonoscopy or patient preference. A medically unfit patient who couldn’t tolerate the conventional laxative bowel preparation may be suitable for a CT with oral contrast (faecal tagging).

Preparing for the test

Stop clopidogrel (7 days) or warfarin (5 days) before the test if instructed, and temporary heparin injection may be necessary

1 week before test
Reduce or stop codeine phosphate, lomotil or immodium
Stop oral iron tablets

48 hours before the test
Avoid fruits, vegetables/salads, skins of potato or tomato, nuts, brown/wholemeal bread or biscuits, shredded wheat, bran or oatmeal
Keep to low residue (or low fibre) diet – for example clear soups, meat and fish of all kinds, rice, pasta, spaghetti, eggs, cheese, white flour bread/biscuits and plain ice cream or jelly

24 hours before the test
No solid food
Plenty of clear fluids – for example tea and coffee (with small amount of milk only), fruit squash, carbonated drinks, and clear jelly or ice cream
Start take laxative medications as per instruction sheet from endoscopy unit – there may be variation of regime between different units, for patients with renal impairment or history of previous poor bowel preparation

On the day of test

No solid food. Clear fluids only
Complete taking laxative medications as per instruction sheet
You should expect frequent bowel actions, and diarrhoea with clear liquid stool
Some abdominal cramping is normal
Blood sugar (BM) is checked in patients with diabetes mellitus
A cannula (small plastic tube) is inserted into a vein
You may be offered disposable colonoscopy shorts to wear

In the endoscopy room

A monitoring probe is attached to a finger, and a nasal cannula is attached to a nostril for oxygen supplementation
Endoscope is introduced into the back passage (rectum)

Drugs commonly used for the test

Injection - Sedatives (benzodiazepine, midazolam)
- Painkiller (morphine, commonly pethidine or fentanyl)
- Buscopan

Patient position(s) during the test

Left lateral (see picture 1)

Picture 1


Occasionally, you may be asked to change your position to help the endoscope advance. This may involve lying on your back (picture 2).

Picture 2


Can I watch the test?

This would be possible as the head position faces the video monitor, and the injection drugs given are aimed at minimal to moderate sedation only.


You won’t feel any pain from having some biopsies taken or polyp removed. Injection of additional sedative or painkiller (morphine) drugs will be given as necessary during the tests for adequate control of pain or discomfort.

Specific complications

The complication of perforation (tear of the bowel lining) occurs infrequently, about 1 in 1000 colonoscopy examination. The risk is higher with polyp removal. The risk of bleeding at the site of biopsy or polyp removal is about 1 in 100-200 examinations.
If any of the above complications occur during the test, endoscopic treatment may be applied. In severe cases, surgery may become necessary.
If there is any concern regarding the above complications straight after the test (in recovery), patients will be admitted for close monitoring and/or treatment.

Flexible sigmoidoscopy

The main differences between a flexible sigmoidoscopy and a colonoscopy are:
1. A flexible sigmoidoscopy test is limited to the left side of the large bowel (colon)
2. It is done mostly for diagnostic reason, normally without sedation although this can be given if requested
3. It doesn’t normally require oral laxative bowel preparation. A phosphate enema (fluid placed in the rectum) is given to clear the bowel on the day of the test.