The split-dose regimen has been shown in multiple studies to be superior to non-split-dose regimen for bowel preparation. How soon before colonoscopy should the second half of the regimen be consumed before colonoscopy? In a meta-analysis by Bucci, C et al, the authors showed that the clinical gain of the split-dose regimen is maintained within 3 hours from the end of the purgative intake, progressively decreases after 4 to 5 hours, and fades after 5 hours (1). The American Society of Anesthesiologists recommends at least a 2 hour fast before sedation.

Take Home Point: The superiority of the split-dose regimen is lost if the second dose is consumed more than 5 hours before the scheduled colonoscopy. The bowel cleansing agent should therefore be finished within 3 to 4 hours of scheduled colonoscopy.



  1. Bucci C, Rotondano G, Hassan C, et al. Optimal bowel cleansing for colonoscopy: split the dose! A series of meta-analyses of controlled studies. Gastrointest Endosc 2014; 80: 566-76.