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BACKGROUND We compared the effectiveness of optional split-dose bowel preparation (SDBP) to mandatory SDBP for morning colonoscopies in usual clinical practice. METHODS Adult patients undergoing outpatient early morning (8:00am-10:30am) and late morning (10:30am-12:00pm) colonoscopies were included. Written BP instructions were provided based on randomization: One group was instructed to take their BP (4L polyethylene glycol solution) as a split-dose (mandatory), while the comparator group was allowed the choice of SDBP or single dose BP administered entirely the day before (optional). The primary endpoint, using non-inferiority hypothesis testing with a 5% margin, was adequate bowel cleanliness measured by the Boston Bowel Preparation Scale (BBPS) and defined by a BBPS score ≥6. RESULTS Among 770 randomized patients, there were 267 mandatory SDBP and 265 optional SDBP for early morning, and 120 mandatory SDBP and 118 optional SDBP patients for late morning colonoscopies with completed data. Optional SDBP was inferior to mandatory SDBP with lower proportion of adequate BBPS cleanliness among early morning colonoscopies [78.9% vs 89.9%, absolute risk difference (aRD)11.0%, 95%CI (5.9%, 16.1%)], but was not statistically different for late morning colonoscopies [76.3% vs 83.3%, aRD 7.1%, 95%CI (-1.5%, 15.5%). CONCLUSIONS Optional SDBP is inferior to the mandatory SDBP in providing adequate BP quality for early morning colonoscopies (8:00am-10:30am), and probably inferior among late morning colonoscopies (10:30am-12:00pm). ClincialTrials.gov number, NCT03650725.