Introduction: We are reporting on our experience with a case involving chronic diarrhea that had been continuing for 14 years following anticancer treatment, in which diarrhea and the nutritional status of the patient were unintentionally improved by administered antihistamine. Subject: A 48-year-old female. Fourteen years ago, surgery, chemotherapy, and total pelvic radiotherapy were carried out for cervical cancer, immediately after which diarrhea continued. A laparotomy was performed 11 years ago for Ileus, and radiation enterocolitis was diagnosed upon pathological examination 8 years ago. One month ago, she experienced swelling of the lower limbs after acute cellulitis and visited our outpatient palliative care. Leaking edema due to low nutrition remained following the remission of lymphedema. Ten days prior to re-examination, a maximum dosage of 20 mg/day of olopatadine was prescribed by another department against urticaria, and at the same time, remission of her diarrhea was observed. Oral administration of folic acid and vitamin was completed by continuing small doses, leading to a rise in CK and disappearance of the edema. Discussion: It may be believed that chronic diarrhea was relieved by olopatadine administered for another purpose due to the inhibitory action of 5-hydroxytryptamine 2A receptor, which is a subfamily of serotinin having histamine 1 inhibitory action and the action to increase the bowel movements.