Background: Irritable bowel syndrome (IBS) is a functional disorder of the gastrointestinal tract with a significant placebo response.

Aim: To conduct a systematic review and meta-analysis examining the magnitude of placebo response rate in treatment trials for IBS.

Methods: MEDLINE, EMBASE and the Cochrane central register of controlled trials were searched to identify randomised controlled trials (RCTs) comparing pharmacological therapies with placebo in adult IBS patients.  Studies reported either global assessment of IBS symptom cure or improvement or abdominal pain cure or improvement.  Data were extracted as intention-to-treat analyses with drop-outs assumed to be treatment failures and pooled using a random-effects model.  Proportion of placebo patients experiencing symptom improvement or resolution was reported with a 95% confidence interval (CI).  Effect of trial characteristics on magnitude of placebo response was examined.

Results: In all, 73 RCTs were eligible, including 8364 patients with IBS allocated to placebo.  Pooled placebo response rate across all RCTs was 37.5% (95% CI, 34.4-40.6%).  Rates were higher in European RCTs, RCTs that used physician-reported outcomes and RCTs using shorter duration of therapy.

Conclusions: Placebo response rates across RCTs of pharmacological therapies in IBS were high.  Future research should identify patient characteristics predicting placebo response.