Colonoscopy Preparation: What to Do, Step by Step

Colonoscopy is the examination of the large bowel from within, using a flexible instrument with a camera at its tip. It is used for both diagnosis and screening, and because polyps that are found can be removed in the same session it is also a therapeutic procedure. This article deals less with the procedure itself than with the preparation that determines its result. Details vary between centres and according to the solution used; the written instructions you are given take precedence.

Why Is a Colonoscopy Performed?

The main reasons are:

• Screening: looking for colon cancer and the polyps that precede it in people without symptoms.
• Assessment of symptoms: blood in the stool, a persistent change in bowel habit, unexplained anaemia, unintentional weight loss, prolonged abdominal pain.
• Follow up: surveillance of people who have had polyps removed or previous bowel surgery.

There is no single figure for the age at which screening starts. In the national screening programme in Turkey colonoscopy is defined from the age of 50; a number of international guidelines have brought the starting point forward to 45 for people at average risk. In those with a family history of colon cancer or polyps, in certain inherited syndromes and in patients with chronic bowel inflammation, screening begins at a younger age and at shorter intervals. The age and interval that apply to you are determined by your doctor according to your history.

Why Does Preparation Matter So Much?

In colonoscopy the image depends on seeing the bowel wall directly. Residue left in the bowel covers part of that wall. Small flat polyps are difficult to detect at the best of times; if residue lies over them they may be missed.

The practical consequence is this: a colonoscopy performed with inadequate preparation does not produce a reassuring result. In that situation the procedure is usually repeated after a shorter interval. Following the preparation in full avoids a second procedure and a further wait.

Days Before: Diet

The general approach is to switch to a low residue diet about three days before the procedure. The aim is to reduce fibrous foods that leave residue in the bowel.

Foods usually to be avoided during this period: fruit and vegetables with skins and seeds, dried pulses, wholemeal bread and bulgur, nuts, small seeded foods such as sesame and poppy, and sweetcorn. Red and purple coloured foods are also limited because they can confuse the image.

Foods usually permitted: white bread, rice, pasta, well cooked meat and chicken, eggs, strained soup, cheese and yoghurt. These lists differ from centre to centre.

The Day Before

On the last day solid food is generally not taken and clear liquids are used instead. A clear liquid is one that light passes through and that leaves no colour: water, weak tea, strained clear broth, and colourless or lightly coloured sweetened drinks. Milk and milky drinks are not clear liquids. Red and purple liquids are not permitted because they can be mistaken for blood in the bowel.

Avoiding dehydration matters at this stage. The cleansing solution causes fluid loss; taking enough clear fluid both improves the effectiveness of the preparation and reduces symptoms such as weakness and headache.

The Cleansing Solution and Split Dosing

Bowel cleansing is achieved with a solution taken by mouth. Products differ in volume and content; your doctor decides which one is used.

The point most emphasised today is splitting the dose: part of the volume is taken on the evening before the procedure and the remainder on the morning of the procedure, a few hours beforehand. Compared with the older practice of taking the whole amount the evening before, cleansing has been reported to be better, particularly on the right side of the colon. If the procedure is scheduled very early in the morning the programme may be arranged differently.

Practical points:

• The solution is taken at the stated intervals and at a steady pace rather than slowly; drinking too slowly reduces its effectiveness.
• Drinking it chilled and using a straw makes the taste easier to tolerate.
• If nausea occurs, pause briefly and then continue; stopping altogether leaves the preparation inadequate.
• The most practical indicator that preparation is adequate is that what is passed becomes a clear yellowish fluid.
• Stay close to a toilet and do not go out once you have started the solution.

Medication: What You Must Discuss With Your Doctor

You need to report all medication you take before the procedure, including herbal products and supplements. The following require separate arrangements:

• Blood thinners: bleeding risk is assessed because polyps may need to be removed. These medicines must not be stopped on your own initiative; whether they are stopped is decided by your doctor, taking your cardiac and vascular condition into account.
• Iron tablets: they leave a dark, sticky residue in the bowel and are therefore usually stopped some time before the procedure.
• Diabetes medication and insulin: doses need adjusting because you will be fasting. Separate advice is given on blood sugar monitoring.
• Blood pressure, cardiac and thyroid medication: these are often continued with a small amount of water, but this too depends on your doctor's instructions.

The Day of the Procedure

The procedure is usually performed with sedation, so most patients do not feel pain and do not remember it afterwards. It commonly takes around half an hour, and longer if polyps are removed.

Because sedation is given, you cannot drive that day; someone needs to accompany you home. It is also advisable not to plan anything requiring important decisions on the same day.

After the Procedure

Bloating and wind pain are common in the first hours, caused by the air introduced during the procedure, and pass quickly. Eating usually returns to normal shortly afterwards.

If a polyp has been removed, additional instructions are given about diet and medication; the tissue removed is sent for pathological examination and the timing of the next colonoscopy is set according to that result.

Heavy bleeding, severe and persistent abdominal pain, a rigid abdomen or fever after the procedure are uncommon but should not be left; contact the centre that performed the procedure if these occur.

Common Mistakes

• Not starting the low residue diet on time.
• Not finishing the whole solution.
• Skipping the dose due on the morning of the procedure.
• Not drinking enough clear fluid.
• Not reporting the medication being taken.

Disclaimer. This article is provided for general information only. It does not constitute medical advice, diagnosis or a treatment recommendation, and it is not a substitute for examination, diagnosis and treatment by a physician. The content is not tailored to any individual and is not intended as advertising. Do not start, change or stop any treatment on the basis of the information here. Consult your own doctor about any decision concerning your health and see a healthcare provider about your symptoms. The site owner accepts no liability for any consequences arising from use of this content. Last updated: 2026-09-02.