Lubiprostone for Functional Constipation in the Under 18 Years Patients

The investigators will enroll 274 subjects who will be randomly divided equally into interventional and control groups. Participants and their parents/legal guardians will be instructed to administer the doses at least 5 hours apart with meals and a large volume of fluid. The control group will receive lactulose 1 mL/kg, once or twice daily (maximum 60 mL/day or Bisacodyl tablet (5 mg/tablet) in a dose of 2 tab/day for \< 12 years or 3 tab/day for \> 12 years. Both arms will receive the treatment for 12 weeks followed by 4 weeks follow-up after the end of treatment (week 16). Participants (and their guardians) will be instructed to regularly document the number of spontaneous bowel motions/week and to define their Bristol chart category for every bowel motion. In addition, the participants will be encouraged to contact the trial team if any adverse events appeared. This is in addition to regular visits at 0, 2, 8, 12 weeks for assessment of safety (efficacy will be assessed at weeks 8, 12, and 16).