For patients with inadequate bowel preparation, current guidelines recommend either remedial measures or rescheduling the endoscopy, but previous surveys have found that approximately 30% of patients do not follow medical advice to reschedule the examination if it is rescheduled, while those who take remedial measures on the same day are more compliant, so remedial measures may be a better option than rescheduling the examination. As for the remedy regimen, it has been demonstrated that oral PEG is superior to the enema regimen in terms of bowel cleansing, but PEG is very resistant to be taken by most patients due to its poor taste and the larger doses required, resulting in poor bowel preparation and poor compliance. In contrast, oral mannitol has the advantages of small dose and good taste, and patient compliance may be higher.

Inclusion Criteria:

• Patients aged 18-75 years with a pre-first colonoscopy bowel cleanliness of less than 2 points in any segment of the bowel.

Exclusion Criteria:

- (1) Pregnant women. (2) History of colorectal surgery. (3) Patients with acute exacerbation of inflammatory bowel disease. (4) Patients with severe medical conditions that make them unsuitable for colonoscopy.

(5) Patients with known allergy to mannitol/PEG. (6) Patients who refuse to sign the informed consent form.