Inclusion Criteria

1. Non-pregnant, non-breastfeeding females;
2. 18-65 years old;
3. Established diagnosis of IBS according to Rome II criteria.
4. No alarm indicators on clinical assessment (weight loss of more than 7 kg, bleeding, unexplained recent rectal bleeding uninvestigated by colonoscopy or double contrast barium enema or flexible sigmoidoscopy for the passed 5 years).
5. Females of childbearing potential who are sexually active must be using an acceptable method of contraception for a period of at least one month before the first dose of study medication, continuing through the duration of the study and for one month after the end of the study. Acceptable birth control methods are: surgical sterilization, oral or implanted contraceptive therapy, intrauterine devices and double barrier methods (diaphragm with spermicidal gel or foam or condoms with contraceptive gel or foam). Post-menopausal is defined as >12 months since last menses. Females who are abstinent may participate if they agree to use a double-barrier method of contraception throughout the study should they become sexually active. .
6. Patients must provide written informed consent prior to inclusion in the study.

Exclusion criteria

1. Patients with abdominal pain and discomfort scores less than 40 mm on a VAS or greater scores on less than 4 days out of 14 days during the run-in period (< 30 % of days during the run-in period).
2. Patients with abdominal pain and discomfort scores more than 60 mm on a VAS on more than 10 days out of 14 days during the run-in period (> 70 % of days during the run-in period).
3. Clinically significant abnormal laboratory values at the screening visit
4. Structural or metabolic diseases/conditions that affect the gastrointestinal system. For screening the Bowel Disease Questionnaire 26 which follows in Appendix 1 will be used.

5. Unable to withdraw medications during run-in period that:

   • Alter GI transit including laxatives, antispasmodics, 5-HT 3 antagonists (e.g. alosetron, ondansetron), 5-HT 4 agonists (e.g. tegaserod), triptans (5-HT 1B or 1D agonists), magnesium or aluminum-containing antacids, prokinetics, erythromycin, narcotics, anticholinergics, loperamide
   • Analgesic drugs including opiates, NSAID, COX 2 inhibitors
   • Inhibiting CYP 3A4 and 2D6 including carbamazepine, glucocorticoids, phenobarbital, phenytoin, rifampin, systemic antifungal drugs (e.g. ketoconazole)
   • Benzodiazepines NOTE: Low stable doses of antidepressants, thyroid replacement, estrogen replacement, low dose aspirin for cardioprotection and birth control pills or depot injections are permissible.
6. History of positive stool cultures for pathogenic ova or parasites or enteric pathogens within the past 3 months, which has not been successfully eradicated.
7. Female patients who are pregnant or breast-feeding.
8. Clinical evidence (including physical exam, ECG, laboratory tests) and review of the medical history) of significant cardiovascular, respiratory, renal, hepatic, gastrointestinal, hematological, neurological, psychiatric, or other disease that interfere with the objectives of the study. The Hospital Anxiety and Depression Scale (HADS)33 will be used to exclude patients with significant affective disorders, as well as to determine anxiety and depression scores at the start of the study.
9. Patients known to be hypersensitive to Asimadoline or opioid agonists.
10. Patients who have been previously exposed to Asimadoline.
11. Patients who have participated in another clinical study within the past 30 days.
12. Patients who are considered by the investigator to be alcoholics not in remission or known substance abusers.
13. Previous gastric or intestinal surgery (except appendectomy, cholecystectomy or hysterectomy).
14. Legal incapacity or limited legal capacity