The primary objectives of this study are to evaluate the clinical safety and tolerability and to assess the efficacy of 100 mg, 200 mg, and 300 mg BID dextofisopam compared with placebo in female outpatients with diarrhea-predominant or alternating irritable bowel syndrome (IBS).

This is a double-blind, randomized , placebo-controlled Phase 2b study of the safety, tolerability, and efficacy of 100, 200 and 300 mg BID dextofisopam in female patients suffering from diarrhea-predominant or alternating-Irritable Bowel Syndrome. (d-IBS or a-IBS.) Approximately 120 patients will be enrolled in each of the 4 arms of the study which will be conducted in up to 70 clinical sites in the USA. The patients will; be stratified by diagnosis.

Outpatient females, 18 to 65 years old will be enrolled in the study if diagnosed with d-IBS or a-IBS after having been screened during up to 17 days prior to enrollment (including colonoscopy if not done within the last 5 years) and found to have no organic disease that might have caused their complaints of abdominal pain or discomfort which when started was associated with a change in stool frequency or form and/or improves with defecation.

Patients will be instructed to take 3 capsules of the blinded study drug twice a day by mouth and to record by an interactive voice response system (IVRS ) any change in their symptoms. Patients who meet all the Inclusion and Exclusion criteria will be evaluated at baseline (one day prior to taking study drug ) including laboratory determinations including a pregnancy test for women of childbearing potential, vital signs, electrocardiogram ( ECG), recording of adverse events(AE's) and filling out the IBS Quality of Life Questionnaire( IBSQOL ), the Work Productivity and Activity Impairment Questionnaire-IBS Version ( WPAI:IBS )and the Hospital l Anxiety and Depression Questionnaire ( HAD).

Enrolled patients will have 12 weeks of double-blind treatment followed by a 28 day post treatment period.

Symptoms will be recorded daily

. Patients will visit the investigating site at weeks 4, 8 and 12 after the baseline visit and the procedures performed at baseline will be repeated A post treatment visit will occur 28 days after discontinuing study drug. Vital signs, AE's, concomitant medications and daily and weekly symptom assessment via IVRS will be recorded.

Ages Eligible for Study: 18 Years - 65 Years, Genders Eligible for Study: Female

Inclusion Criteria:

• 1. Outpatient post-menopausal or no pregnant females,18 to 65 years old
• 2. Irritable bowel syndrome, which meet the Rome III criteria for IBS of the diarrhea-predominant or the alternating subtype
• 3. Able to give informed consent
• 4 Willingness to make daily calls on a touch-tone telephone

Exclusion Criteria:

• 1. Clinically significant abnormality on the screening tests.
• 2. Use of any other investigational drug within 30 days before screening visit.
• 3. Serious underlying diseases, including psychiatric disorders or current history of conditions affecting bowel habits
• 4. Previous treatment with tofisopam Related drugs.
• 5. History or presence of clinically significant medical disease that might compromise the study or be detrimental to the patient, such as
• 6. Subject has exclusively constipation-predominant IBS.