Background:Many studies have reported the use of simethicone before colonoscopy removes bubbles. However, guidelines weakly recommend simethicone administration before colonoscopy. The present study aimed to confirm the advantages of taking simethicone and determine the appropriate time for taking simethicone.Methods:We randomly assigned patients to the following 5 groups according to the administration time: 4 groups were divided based on 2 parameters (the day before and on the day of colonoscopy and before and after bowel cleansing) and the remaining group was the control group. We compared bubble score (BS), number of simethicone solution irrigations when visually obscured, satisfaction score of the endoscopist, insertion time.Results:A total of 204 patients were included in the study. There was a difference in BS according to the timing of simethicone administration (P< .001). The group taking simethicone on the day of the test had a better BS than the group taking simethicone the day before (P< .001). The group taking simethicone on the previous day had a better BS than the control group (P= .001). In the group of taking simethicone on the examination day, the number of irrigations was lower, and satisfaction with the inspector was higher than group of taking simethicone on previous day and control group (bothP< .001). The insertion time showed a non-significantly decreasing trend (P= .417).Conclusion:Administering simethicone reduced bubbles and facilitated effective colonoscopy, especially when administrating it on the day of examination. It needs to be administered on the day of the examination regardless of bowel preparation.