From the Guidelines
Taking two doses of bowel prep at night instead of splitting them between night and morning is strongly discouraged, as it may lead to inadequate bowel cleansing and poor visualization during the colonoscopy, potentially requiring a repeat procedure. The most recent and highest quality study, 1, published in 2025, emphasizes the importance of split-dose regimens for optimizing bowel preparation quality for colonoscopy. This study found that split-dose regimens result in better bowel preparation quality, increased adenoma detection rates, and improved patient tolerability compared to same-day regimens.
The recommended approach is to take the first dose of bowel prep in the evening and the second dose in the morning, ideally 4-6 hours before the colonoscopy, as suggested by 1. Taking both doses at night may result in inadequate cleansing by the time of the procedure, as the effects of the preparation can wear off, leading to poor visualization and potentially requiring a repeat procedure. Additionally, taking both doses close together increases the risk of dehydration and electrolyte imbalances.
If you've already taken both doses at night, it is essential to:
- Drink plenty of clear liquids (unless instructed to be NPO)
- Inform the medical staff about this deviation from the preparation instructions when you arrive for your procedure
- Contact your doctor immediately for guidance, as they may need to adjust the procedure schedule or provide additional instructions to ensure the best possible outcome.
The split-dose regimen is recommended because the second dose closer to the procedure time provides better cleansing of the right side of the colon, as highlighted by 1. This approach has been consistently shown to lead to higher adenoma detection rates and improved bowel preparation quality. Therefore, it is crucial to follow the recommended split-dose regimen to ensure the best possible outcome for your colonoscopy.
From the Research
Bowel Preparation for Colonoscopy
- The effectiveness of bowel preparation is crucial for a successful colonoscopy, and the timing of the preparation is an important factor to consider 2, 3, 4.
- Studies have shown that split-dose regimens, where the bowel preparation is taken in two doses, one the night before and one the morning of the colonoscopy, are superior to day-before preparations 3, 4.
- However, taking two doses of bowel prep before the colonoscopy at night rather than the second dose in the morning may not be the recommended approach, as it may interfere with sleep and work 2.
- A study found that patients who took the entire dose of polyethylene glycol on the previous day had more interference with sleep compared to those who took the split-dose regimen 2.
- Another study found that a split-dose regimen with two doses of sodium phosphate solution achieved better bowel preparation than a single dose or polyethylene glycol solution, but was associated with more side effects 5.
- The American Society for Gastrointestinal Endoscopy recommends a split-dose regimen for bowel preparation, with the second dose taken the morning of the colonoscopy 6.
Potential Risks and Considerations
- Taking two doses of bowel prep at night may lead to poor sleep quality and interference with daily activities 2.
- Sodium phosphate solution has been associated with electrolyte and osmolarity imbalances and nephrocalcinosis, and its use has been questioned in certain patient populations 6.
- Polyethylene glycol may be a safer option for patients with significant renal, heart, or liver disease 6.
Recommendations
- It is recommended to follow the standard split-dose regimen for bowel preparation, with the second dose taken the morning of the colonoscopy 2, 3, 4.
- Patients should consult with their healthcare provider to determine the best bowel preparation regimen for their individual needs and medical history 6.