If you cannot finish the prep

In short

If the prep is not going down, stop, breathe, and work through a small set of steps in order. A short pause, a change of temperature, a different chaser, and slower pacing will rescue most evenings. If those do not work, or if you are vomiting, in serious pain, or genuinely cannot drink any more, call the unit. The number on your discharge sheet is meant for tonight. Half a prep that goes in is far better than no prep, and the unit would rather plan around your situation than discover it at the start of the procedure.

What this page covers

An ordered list of things to try, in the order to try them, when the bowel preparation has stopped going down. What to do if you have vomited. When to call the unit. What the unit will do with the information. What can be salvaged.

Why prep stalls — taste, nausea, fullness, dread
The order of moves to try, from gentlest to most decisive
What counts as a reason to call the unit
What partial prep is worth, and what the unit can do with it
How to think about the rest of the night and the morning

Why the prep stalls

People stop drinking the prep for predictable reasons: the taste becomes intolerable after the first few cups, nausea sets in, the stomach feels full and unwilling to accept more, the pace becomes punishing, or the whole experience produces a quiet refusal that has nothing to do with the body. Often more than one of these is true at once. They are addressable, in different ways, but only if you stop and address them rather than pushing through.

What does not work is faster drinking. Speeding up to be done provokes vomiting and loses the work already done. What also does not work, past a certain point, is heroic stoicism. If you are crying into the cup, you have stopped making progress.

What to do, in order

Stop drinking for fifteen minutes. Set a timer. Do not look at the bottle. Walk into another room. Most stalls resolve at this step, because the stomach simply needed time to empty.
Cool the solution further. Move it from the counter to the fridge, or pour the next cup over a few ice cubes. Cold is the single most reliable taste adjustment.
Change the delivery. Use a wide straw aimed at the back of the tongue rather than the front. If you have been sipping, switch to taking a full cup down in a few swallows and chasing it. If you have been gulping, switch to small, frequent sips.
Reset the palate. Suck on a piece of pale hard candy, a ginger candy, or a slice of lemon for one to two minutes. Rinse your mouth with cold water. Try a sip of a permitted clear liquid you actually like — apple juice, ginger ale, lemon-flavoured sports drink — between cups.
Adjust the pace. If you were drinking a glass every ten minutes, slow to one every fifteen or twenty. The total volume matters; the precise schedule is forgivable.
Address the nausea directly. Open a window. Splash cold water on your face. Walk for two minutes. If you have an anti-nausea medication previously prescribed, this is a reasonable point to take it. Do not start a new medication you have not been prescribed.
Call the unit. If you are still unable to drink after thirty to forty-five minutes of pause and small attempts, telephone the number on your appointment paperwork. Most units have an after-hours line for exactly this. Tell them how much you have drunk, how much remains, what time you started, what time your appointment is, and how you feel. They will tell you what to do next.
Do not skip the second dose. If the first dose is fully on board, the second dose — taken in the early hours of the morning — is the more important of the two for the right side of the colon. Many people who stalled on the first dose manage the second one because their stomach has emptied overnight.
Get whatever fluid you can in. Even if you cannot face the prep itself, keep up clear liquids until your unit's cut-off. Hydration matters in its own right and the unit's plan will be easier to make if you arrive well hydrated.
Sleep, set the alarm, and reassess at the second dose. The night feels long now and short by morning. The decision about whether to attempt the second dose depends on what the unit said on the phone and how the bowel movements look (see how to tell the prep has worked).

If you have vomited

Vomiting a small amount during prep is common and not, by itself, a reason to abandon the schedule. Pause for at least thirty minutes, rinse your mouth, sip a permitted clear fluid slowly, and resume the prep at a slower pace once the nausea has settled.

Vomiting a large amount, vomiting more than once, or vomiting in a way that brings up most of what you just drank is a reason to call the unit before continuing. The unit will weigh how much medication is likely still on board, how much time remains, and whether to switch tactics — for example, dividing the remaining volume into smaller, more frequent doses, prescribing an anti-nausea medication, or rescheduling the procedure for a different prep approach.

If you have severe abdominal pain that does not ease after passing fluid, vomiting that contains blood or coffee-ground-coloured material, signs of severe dehydration (very dark urine, dizziness on standing, palpitations), or chest pain — stop the prep entirely and seek medical attention. These are uncommon but worth knowing about.

When to call the unit

Call the after-hours number if you are unable to keep the prep down despite the steps above; if you have not had any bowel movement two or three hours after the first dose; if your output is still solid or dark brown by the time the first dose is finished; if you are vomiting repeatedly; if you are in severe abdominal pain or have new severe pain; or if you are unsure whether to continue. The phone call is not a failure. It is the system working as intended.

If you genuinely cannot reach the unit and you have a serious symptom — chest pain, faint, severe abdominal pain, blood in vomit — stop trying to finish the prep and contact your local emergency service.

What partial prep is worth

If you have managed roughly half the prep but are unable to finish, your colon will be partially clean — usually adequately on the left side and around the rectum, less reliably on the right. The endoscopist may still be able to perform the procedure, may need to spend longer washing and suctioning, or may decide that the right side cannot be safely inspected and recommend a repeat at a shorter interval. None of these outcomes is catastrophic, and all of them are easier on the team if they know in advance.

If you have managed only a small amount and the bowel movements are still mostly solid or dark, the procedure will likely be rescheduled. This is not a punishment. It is the unit using your day for a procedure it can actually complete safely. The unit may switch you to a different preparation, prescribe an anti-nausea medication, or extend your low-residue diet by a day or two for the next attempt.

What to ask your clinician

What is the after-hours phone number, and what hours does it cover?
If I can drink only part of the prep, at what point should I stop trying and call?
Is there a different prep I could be switched to next time if this one is not working for me?
Can I be prescribed an anti-nausea medication for the next attempt?
If the procedure is rescheduled, how soon can it be redone, and will I need to repeat the entire prep from the beginning?
What does adequate cleansing look like in the toilet bowl, so that I can judge for myself whether the prep is working?

Common worries, briefly addressed

I have lost my nerve and do not want to drink any more.

This is the most common version of the problem and the most underdiagnosed. The honest move is to pause for fifteen or twenty minutes, eat a permitted clear liquid such as warm broth or a yellow ice pop, and then attempt the next cup. Many people who think they have hit a wall find that the wall has moved by the time they look at it again.

If I call, will the procedure be cancelled?

Often not. Many calls end with the unit advising on tactics — switch to small frequent sips, take an anti-nausea medication, drink the second dose as planned and reassess in the morning — rather than rescheduling. The unit prefers a phone call at midnight over a wasted procedure slot at nine in the morning.

I have only managed a third of it. Should I just go to bed?

Call the unit before you decide. The right answer depends on which third, what time it is, and what your appointment is in the morning. Do not unilaterally decide; the unit can tell you in two minutes whether to keep trying, switch approaches, or come in for an assessment.

Can I make up for missed prep with an enema in the morning?

Some units use a morning enema as a rescue, but only on their own instruction. Do not buy one and use it without asking. The procedure-day schedule and any sedation plan depend on what is in your gut, and improvising changes the calculation.

I am exhausted and just want to sleep.

This is allowed. Sleep, set the alarm for the second dose, and address the situation in the morning. Most stalls on the first dose are recoverable.

Sources

American Society for Gastrointestinal Endoscopy and U.S. Multi-Society Task Force on Colorectal Cancer — guidelines on bowel preparation
European Society of Gastrointestinal Endoscopy — bowel preparation guideline
American College of Gastroenterology — clinical guideline on coloscopy preparation
British Society of Gastroenterology — bowel preparation guidance
Canadian Association of Gastroenterology — patient information on coloscopy