['The quality and tolerability of antegrade gut lavage bowel preparation are key elements in the success of population-based colorectal cancer screening.', 'To evaluate cleansing quality and tolerability according to the timing of polyethylene glycol administration in persons undergoing colorectal cancer screening.', 'Participants in colorectal cancer screening were randomized to two groups: a) control group (colonoscopy scheduled at 9-12 h); preparation with polyethylene glycol on the previous afternoon; b) study group (colonoscopy scheduled at 12-15 h): preparation with polyethylene glycol on the morning of the colonoscopy, with the option of a split dose. The quality of cleansing was evaluated with the Boston scale and tolerability through a questionnaire.', 'A total of 282 participants were included: preparation was carried out the day before the procedure in 134 and on the same day in 148, of which 26 received a split dose. Cleansing was adequate in 95% (n=268) of the participants. The quality of cleansing was higher in the study group (P=.045). The interval between the end of administration and the beginning of the procedure was inversely correlated with the Boston scale score (P=.036; r=-0.125). Tolerability was unrelated to the time of administration (P>.2). Acceptance of the timing of administration was lower in the study group than in the control group (26% vs 10%, respectively; P=.001).'] Is [ Timing of polyethylene glycol administration a key factor in the tolerability and efficacy of colon preparation in colorectal cancer screening ]?