title: Defining the predictors of the placebo response in irritable bowel syndrome.
abstract: OBJECTIVE
  We sought to determine the components of irritable bowel syndrome (IBS) clinical trials that correlate with higher levels of placebo response.
  
  
  METHODS
  We performed a systematic review of placebo-controlled trials in patients with IBS to assess which variables correlate with a higher placebo response. Placebo responses for global symptom improvement and for decreased abdominal pain were assessed. Univariate and multiple linear regression analyses were conducted.
  
  
  RESULTS
  Higher rates of global improvement correlated with frequency of administration of study intervention ( r = .31, P = .03), duration of the study ( r = .28, P = .04), and overall treatment effect of the active agent being studied ( r = .33, P = .02). Higher rates of decreased abdominal pain correlated with the frequency of intervention ( r = .39, P = .02) and overall treatment effect ( r = .40, P = .01), whereas lower placebo response rates correlated with year of the study ( r = -.36, P = .03), median age ( r = -.38, P = .04), and duration of study run-in period ( r = -.33, P = .04). On multivariate analysis, global improvement in the placebo group was associated significantly with intervention frequency ( P = .0079), overall treatment response ( P = .0031), and parallel study design ( P = .0044). Decreased abdominal pain was associated significantly with frequency of intervention ( P = .0061) and overall treatment response ( P = .0128).
  
  
  CONCLUSIONS
  In IBS studies, higher placebo response rates correlated with frequency of the intervention and with overall treatment effect of the active agent being studied. In designing IBS trials, it may be possible to minimize placebo response by less frequent dosing. In treating patients with IBS, it may be possible to harness the placebo response and maximize therapeutic response rates by more frequent dosing.
authors.affiliation: Department of Internal Medicine, University of Manitoba, Manitoba, Canada.
authors.forename: Marshall
authors.initials: M
authors.lastname: Pitz
authors.forename: Mary
authors.initials: M
authors.lastname: Cheang
authors.forename: Charles N
authors.initials: CN
authors.lastname: Bernstein
mesh_terms: D002986:Clinical Trials as Topic; D004334:Drug Administration Schedule; D005765:Gastrointestinal Agents / Q000008:administration & dosage*; D006801:Humans; D043183:Irritable Bowel Syndrome / Q000188:drug therapy*; D015990:Placebo Effect
chemical_list: D005765:Gastrointestinal Agents