The purpose of this study is to compare the safety and effectiveness of as needed versus fixed dosing of an investigational medication for women with severe diarrhea-predominant Irritable Bowel Syndrome (IBS) who have failed conventional therapy.

Inclusion Criteria:

• Diagnosed with severe diarrhea-predominant Irritable Bowel Syndrome (IBS).
• Failed conventional therapy.
• Willing to make daily calls on a touch-tone telephone.

Exclusion criteria:

• History of or current chronic or severe constipation.
• Bloody diarrhea, abdominal pain with rectal bleeding.
• Thrombophlebitis.
• Abnormal thyroid stimulating hormone (TSH) value.
• Alcohol and/or substance abuse within past two years.
• Pregnant or lactating.
• History/treatment of malignancy within past five years.