Optimal bowel preparation is essential for colonoscopy efficacy and safety. Mucosal visualization during colonoscopy is often limited by residual stool, bubbles, bile, intraluminal fluid, and debris, which increase the risk of missing flat adenomas or other small lesions.Therefore, intestinal preparation is necessary to remove residual materials prior to endoscopy.

A combined agent, low-dose PEG with ascorbic acid (PEG-Asc), is one low-volume solution commonly used in Korea (Coolprep®; TaeJoon Pharmaceuticals, Seoul, Korea). However, practitioners have noted an increased incidence of bubble formation with this preparation method.

To the investigators knowledge, no previous study has assessed colon preparation in patients administered simethicone.

The purpose of this study was to compare the quality of bowel preparation and compliance between PEG-Asc and PEG-Asc with simethicone. The effectiveness of adding simethicone as an antifoaming agent to improve bowel cleansing for colonoscopy was evaluated in terms of bowel preparation scale and bubble score, and the compliance of both patients and endoscopists was also investigated using a questionnaire.

1. Study design: endoscopist-blinded, prospective, randomized controlled trial

2. Subjects

   - A single-center, randomized, observer-blinded study was performed at Korea University Hospital in Anam. Outpatients were prospectively enrolled. Each group of the patients will receive PEG-Asc or PEG-Asc with simethicone before colonoscopy.

3. Sampling design: Consecutive recruitment of consenting patients

4. Variables Predictor

   1. group 1 (PEG-Asc) received 1 L solution of PEG-Asc at 7 p.m the evening before colonoscopy and another 1 L solution of PEG-Asc at 5 hours before procedure;
   2. group 2 (PEG-Asc with simethicone) received 1 L solution of PEG-Asc at 7 p.m the evening before colonoscopy and another 1 L solution of PEG-Asc at 5 hours before procedure;Two packs (200 mg/10 mL each) of simethicone (400 mg) was mixed with last 500 mL of additional clear fluid.

5. Primary Outcome: Quality of bowel preparation [Boston Bowel Preparation Scale, bubble score]

6. Secondary Outcome: Patients and endoscopists' compliance[Tolerability, palatability of patient, fatigue score of endoscopist]

Inclusion Criteria:

• Male or female patients,
• aged between18 and 80 years undergoing elective outpatient colonoscopy were eligible for the study

Exclusion Criteria:

• patients who had chronic kidney disease,
• severe heart failure(New York Heart Association [NYHA] class III or IV)
• uncontrolled hypertension (systolic pressure ≥170 mm Hg, diastolic pressure ≥100 mm Hg)
• severe constipation
• any bowel resection
• significant gastroparesis, or
• suspected bowel obstruction or perforation.