Bowel preparation, or the prep, is the process of emptying and cleaning the colon before a colonoscopy. It combines a short period of diet changes with a prescribed laxative solution that flushes the colon clear.
It is fair to say the prep is most people's least favorite part of a colonoscopy. It is also the part that matters most: a clean colon lets your gastroenterologist see small polyps and subtle changes, while a poorly cleaned one can hide them or mean the exam has to be repeated.
Changing what you eat
A few days before the exam, your instructions may ask you to avoid high-fiber foods such as seeds, nuts, corn, and raw vegetables.
The day before, you will switch to clear liquids only: water, broth, clear juices without pulp, plain gelatin, and coffee or tea without milk. Skip anything colored red or purple, which can be mistaken for blood during the exam, and avoid alcohol.
Taking your prep
Your prescribed prep is usually taken in two parts: the first dose the evening before, and the second dose several hours before your procedure. This split schedule cleans the colon best, even though it can mean an early alarm; set one, and take the second dose on time.
Expect frequent, urgent, watery bowel movements within an hour or so of starting, so plan to stay near a bathroom. Chilling the solution, drinking it through a straw, and sipping steadily rather than gulping all make it easier. If you feel nauseated, slow down, take a short break, and then continue.
Keep drinking the clear liquids your instructions allow so you stay hydrated; the prep works best, and you will feel best, when you do. Stop all liquids, including the prep, at the cutoff time your care team gives you before the procedure.
Common questions
How do you know it worked? By the end, your bowel movements should look like clear or pale yellow liquid. If they are still brown or cloudy, finish the remaining prep and mention it when you arrive.
What about your medicines? Ask your care team which morning medicines to take with a small sip of water. Blood thinners and diabetes medicines often need special timing, so bring them up when your colonoscopy is scheduled rather than the night before.
And if you cannot finish? If you vomit repeatedly or truly cannot drink any more, contact your care team before your appointment instead of simply skipping doses; they will tell you how to proceed.
Questions about your care?
This page is general education, not personal medical advice. Your care team is happy to talk through what it means for you.
Preparing for a procedure? See preparation instructions