Sometimes, the preparation solution to clear the bowels before a colonoscopy does not work, or only partially works. A person can speak with their doctor about ways to make the prep more effective.

Between 10–25% of colonoscopies have inadequate preparation. Because the doctor needs to be able to see the lining of the colon in its entirety, the bowels must be empty for the procedure to be successful.

If the prep does not work, the doctor could miss important details by performing the colonoscopy without the bowel being empty, the procedure may need repeating or rescheduling.

In this article, we discuss what to do if the colonoscopy prep is not working. We also provide tips on how to drink all the prep and explain the other steps a person can take to prepare for a colonoscopy.

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The body’s response to the colonoscopy prep drink can vary among individuals, as well as the type of prep they take. When people use MiraLax, bowel movements typically begin 1 hour after drinking the solution.

However, if bowel movements do not begin when a person expects, this does not necessarily mean the solution is not working, particularly if a person is taking their prep in two separate doses.

A person should consult their doctor if they have concerns the prep is not working. In the meantime, a person may need to continue to consume the prep and drink plenty of clear liquids.

A doctor may suggest taking an additional laxative product. If a person is experiencing severe constipation, they may require an enema.

It may be challenging to drink all of the colonoscopy prep, especially if a person does not like the taste. However, people can make the prep more drinkable by:

  • Splitting the prep: Rather than drinking the colonoscopy prep the evening before the colonoscopy, doctors may advise that people split it into two doses, taking some the night before and some more 3–8 hours before the procedure.
  • Using a straw: Drinking the mixture with a straw can help it bypass the taste buds, so people are less likely to taste it.
  • Drinking the prep in sips: Instead of drinking the entire dose of prep in one or two sittings, people may find it easier to take sips and drink other clear liquids in between.
  • Chilling it: Anecdotal evidence suggests that the prep drink tastes worse when warm, so it may help to refrigerate the prep or drink it over ice.

It is important to discuss these options with a doctor before trying them to make sure that they do not affect the specific prep that the person is taking.

A person needs to take additional steps before a colonoscopy to give the prep drink the best chance of emptying the bowels before the procedure. These include:

Reviewing medications and supplements

Some medications and supplements, such as iron tablets, can cause constipation. A doctor may recommend temporarily stopping any of these before a colonoscopy so they do not interfere with preparation.

Making dietary changes

Doctors may recommend that people switch to a low fiber diet several days before the colonoscopy. People on a low fiber diet tend to avoid:

  • whole grains, such as brown rice or oats
  • foods that contain whole grain flours, such as brown bread
  • nuts and seeds
  • raw vegetables
  • raw and dried fruits
  • the skin of fruits and vegetables
  • fruit juice with pulp

The day before a colonoscopy, people will need to change to a clear liquid diet. A clear liquid diet allows:

If a person is unsure whether they can consume a specific product, they should check with a doctor.

People must take particular care to avoid anything containing purple, blue, or red food coloring, as these can resemble blood in the colon and may give misleading results.


People should stay close to a bathroom during colonoscopy preparation, even if the prep does not appear to be working yet. The prep could take slightly longer than usual to work, but then suddenly be effective. Wearing loose and elasticated pants without buttons may save time in case of urgency.

Colonoscopy prep liquids will cause diarrhea, which may lead to soreness around the anus. Liberally applying petroleum jelly or a similar product may help reduce discomfort. People could also use flushable baby wipes or moistened toilet tissues instead of toilet paper to reduce irritation.

If colonoscopy prep is not working after the second dose, people should contact their doctor to tell them.

The doctor may have provided a leaflet with the prep drink that details what an individual should do if they have not had a bowel movement after consuming all of the colonoscopy prep drink. If they do not have any information about this, people should consult the doctor rather than guessing.

It is important to speak with a doctor before trying anything else to help encourage bowel movements, such as additional medications.

Constipation can sometimes occur after a colonoscopy. Other minor complications can include:

  • bloating
  • discomfort
  • abdominal pain

A small 2021 study suggests that these symptoms may occur due to the colonoscopy disrupting gut flora. The symptoms should get better on their own, but it may take some time to settle down.

If the symptoms do not improve, they get worse, or they are severe, speak with a doctor right away.

If a person’s colonoscopy prep is not working, it may be taking longer to have any effect than usual. Continuing to drink water, stick to a clear liquid diet, and avoiding things that may cause constipation may allow the prep to work.

If the second dose of the prep does not work, or a person’s procedure is happening soon but the colon is not fully prepared, they should contact their doctor. The procedure may need to take place on another day. Alternatively, if there is time, a doctor may suggest trying an additional laxative.