Methods for treating constipation-predominant irritable bowel syndrome comprise administering to a patient in need thereof a polymeric proanthocyanidin composition from a Croton species or Calophyllum species in an amount sufficient to treat constipation-predominant irritable bowel syndrome (c-IBS). Treatment of c-IBS includes the treatment of the constipation component of c-IBS as well as the pain and abdominal discomfort associated with c-IBS. In one embodiment, the polymeric proanthocyanidin compound is crofelemer. In addition, methods for treating alternating constipation-predominant/diarrhea-predominant irritable bowel syndrome are an alternative embodiment of the invention.