A Clinical Study to Evaluate the Usefulness of Concomitant Itopride Hydrochloride (Ganaton) for the Treatment of Nausea Associated with Lubiprostone (Amitiza)

INTERVENTION: Intervention type:DRUG. Intervention1:Ganaton, Dose form:TABLET, Route of administration:ORAL, intended dose regimen:The dosage is one tablet three times daily, taken orally before meals. Intervention2:Amitiza Capsules 24 microgram, Dose form:CAPSULE, Route of administration:ORAL, intended dose regimen:The dosage is 24 microgram of lubiprostone twice daily, taken orally after breakfast and evening meal.. Control intervention1:Ganaton Placebo Tablets, Dose form:TABLET, Route of administration:ORAL, Intended dose regimen:The dosage is one tablet three times daily, taken orally before meals., Control intervention2:Amitiza Capsules 24 microgram, Dose form:CAPSULE, Route of administration:ORAL, Intended dose regimen:The dosage is 24 microgram of lubiprostone twice daily, taken orally after breakfast and evening meal.. CONDITION: chronic constipation PRIMARY OUTCOME: To evaluate the incidence of nausea events during the 14‐day period. Timepoint:Day15. SECONDARY OUTCOME: Nausea‐related dropout rate, Nausea severity level, Clinical efficacy of AmitizaR in treating chronic constipation, Drug safety. Timepoint: day15. INCLUSION CRITERIA: 1. Patient diagnosed with chronic constipation. 2. Male or female outpatient between 20 and 75 years of age at the time consent is given 3. Patient who has never been on AMITIZA 4. Patient agreed to achieve satisfactory compliance to the regimen of medication stipulated in written form, and to undergo all medical examinations, report symptoms to the investigator, and keep the patient's diary through study inclusion period, as per required in study protocol.