Multi-centre, double-blind, randomised, placebo-controlled, parallel group study.

Patients will be treated for 28 days, with clinical assessments carried out over 2 visits. Follow-up will be performed via telephone at a specific interval during the study to determine the outcome of unresolved AEs, any new drug-related AEs or any pregnancies. During the run-in period and the treatment phase, patients are prohibited from taking prescription or over the counter (OTC) laxative medication or constipating medications. Prescription and OTC medications not related to constipation are permitted and must be fully documented, but any changes (e.g., dosage, regimen) are to be limited as much as possible.

Inclusion Criteria:

• Male and females aged 18 to 80 years inclusive.
• Written informed consent obtained.
• Females of child-bearing potential must employ an adequate method of contraception and must undergo a pregnancy test at Randomisation Visit.
• Willing, able and competent to complete the entire study and comply with study instructions.
• Patients > 50 years old have to have undergone a colonoscopy or computerised tomography (CT) after the onset of their IBS-C symptoms or within the last 5 years.
• Patient has diagnosed IBS-C using Rome III criteria for the last 3 months, with symptom onset at least 6 months prior to diagnosis.

Exclusion Criteria:

• History or evidence of organic disease in the large bowel, intestinal perforation or obstruction due to structural or functional disorder of the gut wall, ileus, severe inflammatory conditions of the intestinal tract such as Crohn's disease, complicated diverticulitis, ulcerative colitis, toxic megacolon and occlusive or subocclusive syndrome.
• Abdominal pain of unknown cause, not related to IBS.
• Previous major abdominal surgery.
• IBS subtype other than IBS-C.
• Known allergy to polyethylene glycol (PEG) 3350 or known hypersensitivity to any of the active substances.
• Laxative, drug or alcohol abuse (recent history or within previous 12 months).
• Pregnant or lactating females.
• Severe or acute disease within 2 weeks prior to the start of the study.
• Patients with type I or II diabetes.
• Use of other investigational drugs, prescribed or OTC medications affecting gastrointestinal function such as anticholinergics, prokinetics, drugs affecting motility, anthraquinones, opioids,ondansetron or other 5-HT3 antagonists.
• Incomplete Patient Diary Card during the run-in period.
• The occurrence of diarrhoea* during the run-in period.
• Patients with abnormal laboratory tests, proctoscopy /abdominal ultrasound, colonoscopy, sigmoidoscopy or CT that requires further investigation.
• Patients with any condition, which, in the Investigator's may put the patient at significant risk, may confound the study results, or may interfere significantly with the results of the study.
• Participation in another clinical study of drugs or devices parallel to or less than 1 month before study entry, or previous participation in this study.
• Employees of the Investigator or study site with direct involvement in the proposed study or other studies under the direction of that Investigator or study site, as well as family members of the employees or the Investigator.