Previous with methylene blue suggested that oral administration may be superior to conventional spray catheters. The main advantages claimed were better staining due to longer exposure to the dye, more uniform staining due to normal position of the intestines (during conventional chromocolonoscopy, patients are lying on one side), and a quicker and more comfortable procedure for the patients and endoscopy personnel.

Using food-grade dyes would enable higher dye loads and lower systemic side effects.

Food grade dyes will be administered orally during preparation with KleanPrep (total amount of 4 liters) in patients undergoing screening colonoscopy.

On day 1 patients will drink 3 l of the preparation solution, the last liter together with the last dose of dye will be administered 5 hours before scheduled colonoscopy.

Primary endpoints:

• quality of mucosal staining of colon (assessed by questionnaire)

Secondary endpoints:

• adenoma detection rate
• adverse events
• time of colonoscopy

Inclusion Criteria:

• Healthy males and post-menopausal females aged 18 to 75 y and scheduled for screening colonoscopy
• body weight ≥60 kg
• good health based on medical history, physical examination, a 12-lead electrocardiogram (ECG) and routine haematology and blood chemistry tests
• ability to understand and comply with the protocol
• written informed consent

Exclusion Criteria:

• standard criteria for bioavailability estimation of new drugs, namely (i) intake of any medication, (ii) a history of drug, caffeine (>5 cups coffee/tea/day) or tobacco (≥10 cigarettes/day) abuse, (iii) history of alcohol consumption in excess of two drinks per day in males and one drink per day in females
• known or suspected hypersensitivity to food colourants,
• GI obstruction or perforation
• serious cardiovascular, renal or hepatic disease
• prolonged prothrombin time, elevated INR (international normalized ration)
• elevated serum creatinine
• any other severe underlying medical condition