This is a prospective, randomized, single-blind study designed to evaluate the influence of individual approach on bowel preparation quality for colonoscopy. The investigators compare split-dose polyethylene glycol solution (PEG, Fortrans, Beaufour Ipsen) to modified split-dose PEG regarding predictors of inadequate preparation for colonoscopy.

Up to 22% of patients are not good prepared for colonoscopy and it is caused of aborted procedures and increased costs. There are identified predictors of inadequate bowel preparation before colonoscopy, but we still don't know if an individual approach improve the quality of preparation.

Inclusion Criteria:

• men and women more than 18 years of age undergoing colonoscopy for routine clinical indications
• provided written informed consent

Exclusion Criteria:

• lack of written informed consent or inability to provide informed consent or refusal to consent to the study
• pregnancy and breast feeding
• known allergy to PEG or bisakodyl
• presence of serious conditions such as severe cadiac, renal or metabolic diseases, major psychiatric illness or end-stage cancer disease requiring taking narcotic drugs
• colonoscopy performed under conscious sedation