The investigators will enroll subjects 8 - < 18 years of age, fulfilling Rome IV criteria for functional constipation.

Participants will be randomly assigned to either Lubiprostone treatment (study group), or the control group (will receive either lactulose or Bisacodyl tablets).

safety and efficacy will be assessed.

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).

Inclusion Criteria:

• Patients 8 - < 18 years of age who have a confirmed diagnosis of Pediatric functional constipation according to the Rome IV criteria, who give written informed consent personally or from their legal guardians.
• Discontinuation of any medication affecting gastrointestinal (GI) motility at least 2 weeks before starting the treatment allocation.
• Patient's daily diary that indicates an average of < 3 weekly spontaneous bowel movements (SBMs), with ≥ 25% of SBMs involving at least some straining and/or a 5-point modified Bristol Stool Form Scale type 1 or 2.
• Patients who completed the study protocol.

Exclusion Criteria:

1. If the patient's constipation is attributed to any of the following: physical, mental, or cognitive illness, inflammatory bowel disease, medication, anatomical, neurological, endocrine, or metabolic factors.
2. If the patient is a candidate for or underwent abdominal surgery, or has any condition other than constipation that could affect gastrointestinal motility or defecation.
3. Patients suffering from Hirschsprung's disease.
4. Patients experiencing any alarming signs e.g. unexplained significant weight loss.
5. Untreated fecal impaction at the time of enrollment.

patient personal and identification data will not be available for sharing