The short answer: Keep following your written instructions and call your endoscopy unit. Do not add another laxative on your own. The GoLYTELY label says the first bowel movement should come about one hour after you start drinking. If nothing has happened well past that point, or your output is still solid or brown after you finish, the unit needs to know before you travel in.
How long does colonoscopy prep take to start working?
Only one of the three labels we read gives a time. The GoLYTELY label says "the first bowel movement should occur approximately one hour after you start drinking the solution." GoLYTELY is a polyethylene glycol (PEG) solution that you drink at 8 ounces every 10 minutes.
The Suprep label and the Sutab label do not state when the first bowel movement should come.
Patient pages from the two endoscopy societies are also general. The American College of Gastroenterology (ACG) describes prep as a laxative that causes "temporary diarrhea for several hours." The American Society for Gastrointestinal Endoscopy (ASGE) says to expect "multiple loose bowel movements" during the prep.
A slow start does not always mean a failed prep. But a long delay is a known warning sign. A 2024 meta-analysis of 154 studies found that a longer time from prep to first bowel movement predicted an inadequate prep.
What is safe to do while you wait?
The labels describe a small set of safe steps.
- Keep to the schedule if you feel well. The GoLYTELY label says "rapid drinking of each portion is better than drinking small amounts."
- Expect some bloating. The GoLYTELY label says "you may experience some abdominal bloating and distention before the bowels start to move."
- Slow down or stop if the discomfort is severe. The GoLYTELY label says to "slow or temporarily stop" drinking and contact your healthcare provider if severe discomfort or distention occurs.
- Pause the water with Sutab if you have symptoms. The Sutab label says to "pause or slow the rate of drinking the additional water until symptoms diminish" if you have nausea, bloating, or cramping.
- Wait before the second dose if your stomach is upset. The Suprep and Sutab labels both say to wait to take the second dose until your stomach feels better.
- Drink clear liquids. The ASGE page says to drink more fluids than you usually do during prep.
All three labels say the same thing about other laxatives: "Do not take other laxatives" while you take the prep. They give a reason. Stimulant laxatives such as bisacodyl, taken with these products, "may increase the risk of mucosal ulceration or ischemic colitis." Ischemic colitis is injury to the colon from reduced blood flow.
If nausea or vomiting is the reason the prep is not going down, read our guide to vomiting during colonoscopy prep.
When should you call the endoscopy unit?
Call the number on your prep instructions in any of these cases:
- You have had no bowel movement well past the time your label or your unit told you to expect one.
- You cannot finish the prep. The ASGE page says to "call your doctor and ask what to do if you are not able to complete the bowel prep as advised."
- You finished the prep and your output is still solid or brown.
- You have severe bloating, distention, or abdominal pain. The GoLYTELY label says to contact your healthcare provider.
The ASGE page says the liquid you pass after the full prep should be "watery or yellow and clear enough to see through." Our guide to stool color when colonoscopy prep is done shows what that looks like.
Do not cancel the exam yourself because of what you see in the toilet. The 2025 recommendations from the US Multi-Society Task Force on Colorectal Cancer call patient self-assessment of prep quality "unreliable." They cite a study of 429 patients. Of the 52 who reported brown liquid or solid stool just before the exam, 42% had a prep the endoscopist rated adequate after washing and suctioning.
The 2025 document updates the 2014 edition.
Who is at higher risk of an inadequate prep?
The 2025 Task Force document lists risk factors with odds ratios. An odds ratio above 1 means higher odds of an inadequate prep. The largest ones on its list are:
- Cirrhosis: 3.4
- Parkinson disease: 3.2
- Dementia: 3.0
- Tricyclic antidepressant use: 2.0
- Diabetes: 1.8
- Opioid use: 1.7
- Gastroparesis (slow stomach emptying): 1.6
- Constipation: 1.3
- High blood pressure: 1.3
The 2024 meta-analysis pooled 154 studies with 358,257 participants. It found many of the same factors and added calcium channel blockers, a common class of blood pressure medicine. Our sister site hypertension.md covers those drugs. Do not stop a prescribed medicine because of this list. Ask your prescriber.
What happens during the prep matters more. A second 2024 meta-analysis of 125 studies found three strong predictors: a solid or brown liquid last stool, incomplete prep intake, and incorrect diet restriction.
If you have one of these risk factors, tell the unit when you book.
What can the endoscopy unit do if the prep falls short?
The 2025 Task Force document describes several options.
- Look before canceling. When a patient reports dark output or an unfinished prep, the Task Force suggests passing the scope to the sigmoid colon to confirm the problem before stopping.
- Wash and suction during the exam. In one study of 525 patients, 75% of those with a poor or fair prep at the start were converted to good or excellent by washing and suctioning.
- Give an enema through the scope. The exam can then resume later the same day. The Task Force reports success rates from 53% to 100%.
- Prescribe more oral prep. The exam follows the same day or the next day. In one randomized trial, an additional 2 liters of PEG by mouth produced an adequate prep in 82% of patients, compared with 53% for a 1-liter PEG enema.
- Reschedule. If the prep cannot be rescued, the Task Force recommends a repeat colonoscopy within 12 months for screening or surveillance exams. If the exam was ordered for alarm symptoms or a positive stool test, it should happen as soon as possible.
For the next attempt, the Task Force recommends changes to the instructions. Options include clear liquids only on the day before, treatment of constipation, and a high-volume prep.
The ASGE page states the cost of a poor prep plainly. If the colon is inadequately cleansed, "your doctor may recommend repeating a colonoscopy sooner." Our guide to what happens after an incomplete colonoscopy covers the follow-up.
Which warning signs need urgent care?
A prep that does not move can be a sign of a blockage, although this is not the usual cause. All three labels list bowel obstruction and ileus (a bowel that moves too slowly) as reasons not to take the product.
The labels and MedlinePlus name these signs:
- Severe abdominal pain or rectal bleeding. All three labels say to tell your healthcare provider right away.
- Signs of dehydration. All three labels list vomiting, dizziness, urinating less often than normal, and headache. They say to tell your healthcare provider right away.
- Seizures or an abnormal heartbeat. The labels warn that fluid and salt changes can cause both.
- Confusion or loss of consciousness. The MedlinePlus page on dehydration says to call 911 for a loss of consciousness or any change in alertness, such as confusion or seizures.
If you cannot reach the unit and you have one of these signs, go to an emergency department.
The bottom line
The GoLYTELY label says the first bowel movement should come about one hour after you start. The Suprep and Sutab labels give no time. If the prep is slow, keep drinking as instructed, slow down for severe bloating, and take no other laxative. Call the unit if nothing happens, if you cannot finish, or if the output stays solid or brown. If the unit cannot rescue the exam, the Task Force recommends a repeat within 12 months.
Last updated: October 2026. This article is for informational purposes only and does not constitute medical advice. Follow the prep instructions from your own endoscopy unit, and call the unit or your prescriber before you change how you take a bowel prep.