This is a randomized double-blind placebo-controlled pilot study that will use validated bowel questionnaires to evaluate the effects of ranolazine administered orally twice daily in patients with diarrhea predominant IBS (IBS-D). The study will consist of a 2 week run in period, followed by 4 weeks of treatment period with oral ranolazine 1000 mg twice daily. Primary endpoint of the study will be the average Bowel Symptom Scale (BSS) scores for diarrhea and adequate relief of IBS pain and discomfort are secondary end points.