Prospective, randomized, single-blind, non-inferiority trial in adult patients undergoing screening or surveillance colonoscopy. Eligible participants were randomly assigned, according to a computer-generated list with an allocation ratio of 1:1, to receive either asymmetric or symmetric split-dose regimen of a low-volume preparation by investigators who were not involved in the enrolment process. Detailed written instructions were administered to all patients who accepted to participate. A low-fiber diet was prescribed three day before colonoscopy; participants in both groups were instructed to have a light lunch on the day before the colonoscopy and only clear liquid were allowed on the day of the exam. All endoscopic procedures were scheduled between 9:00 AM and 1:00 PM.