Placebo effect in patients with diarrhea-type irritable bowel syndrome: a literature review of randomized, placebo-controlled trials

Review question / Objective: The aim of this systematic review is to explore the magnitude of the placebo effect in randomized controlled trials for diarrhea-type irritable bowel syndrome and to understand possible relevant factors that affect the placebo effect. Condition being studied: Irritable bowel syndrome is a chronic functional gastrointestinal disorder characterized by abdominal pain related to defecation and a change in frequency and form of stool. Epidemiological study indicates that the prevalence of irritable bowel syndrome in different countries is high. It is estimated conservatively that direct costs related to irritable bowel syndrome causes a huge economic burden in the United States. In the latest Rome IV criteria, irritable bowel syndrome is divided into 4 subtypes based on abnormal bowel habits: irritable bowel syndrome with predominant constipation, irritable bowel syndrome with predominant diarrhea, irritable bowel syndrome with mixed bowel habits, and irritable bowel syndrome unclassified. Regarding treatment for irritable bowel syndrome, there is no cure or curative treatment. Any agent should be compared with placebo to identify its efficacy. In fact, the placebo response rate of irritable bowel syndrome is high. However, the placebo response rate of IBS-D and the moderators of the magnitude of the placebo response rate are not clear.