The objective of this randomized triple blind controlled trial is to compare the effects of Doxepin and Nortriptyline on diarrhea predominant irritable bowel syndrome. In this stuy, 75 patients with IBS are randomly assigned to receive Doxepin , Nortriptyline or placebo, a tablet per day, orally, for two months. Abdominal pain, mucus in the stool, incomplete evacuation, and bloating are measured and compared between groups at the baseline and one and two month after the intervention.

Inclusion Criteria:

• Presence of diarrhea predominant irritable bowel syndrome according to ROME criteria

Exclusion Criteria:

• Gastrointestinal bleeding
• More than 5% weight loss in the last 6 months
• Presence of any finding in favor of organic disorders in the lab tests or organic disorder in colonoscopy of high risk patients