Irritable bowel syndrome (IBS) is a functional gastrointestinal tract disorder characterized by recurrent abdominal pain or discomfort, where the onset is associated with either a change in form of stool or its frequency and is often improved with defecation. Alosetron, a 5-hydroxytryptamine-3 (5-HT3) receptor antagonist, was an effective drug in treating women with diarrhea-predominant IBS. However, the drug was removed from the United States market because of its association with ischemic colitis and serious complications related to severe constipation. Presented here is a case report of a 66-year-old woman with a history of panic disorder and major depression and a 1-year history of IBS-mixed type, which she reported to be "worsened by panic attacks." On the basis that mirtazapine is a potent 5-HT3 receptor antagonist and has demonstrated pain relief from somatic symptoms, we treated our patient with mirtazapine, which seems to have decreased her diarrhea and constipation symptoms, and her psychopathological symptoms.