Demographic data including age, sex, smoking and alcohol habits, comorbidities, education level, family income, time of using WhatsApp of the eligible subjects will be collected after getting their written consent. Subjects will then be randomized to either standard group or WhatsApp group. The randomization will be based on whether the date of making appointment is an even (assign to WhatsApp group) or odd number (assign to standard group). As a usual practice, all subjects will receive a verbal explanation of bowel preparation by a healthcare professional on the day of appointment making. A detailed written bowel preparation instruction will be also given to subjects.