title: The placebo effect in irritable bowel syndrome trials: a meta-analysis.
abstract: BACKGROUND
  Despite the apparent high placebo response rate in randomized placebo-controlled trials (RCT) of patients with irritable bowel syndrome (IBS), little is known about the variability and predictors of this response.
  
  
  OBJECTIVE
  To describe the magnitude of response in placebo arms of IBS clinical trials and to identify which factors predict the variability of the placebo response.
  
  
  METHODS
  We performed a meta-analysis of published, English language, RCT with 20 or more IBS patients who were treated for at least 2 weeks. This analysis is limited to studies that assessed global response (improvement in overall symptoms). The variables considered as potential placebo modifiers were study design, study duration, use of a run-in phase, Jadad score, entry criteria, number of office visits, number of office visits/study duration, use of diagnostic testing, gender, age and type of medication studied.
  
  
  RESULTS
  Forty-five placebo-controlled RCTs met the inclusion criteria. The placebo response ranged from 16.0 to 71.4% with a population-weighted average of 40.2%, 95% CI (35.9-44.4). Significant associations with lower placebo response rates were fulfillment of the Rome criteria for study entry (P=0.049) and an increased number of office visits (P=0.026).
  
  
  CONCLUSIONS
  Placebo effects in IBS clinical trials measuring a global outcome are highly variable. Entry criteria and number of office visits are significant predictors of the placebo response. More stringent entry criteria and an increased number of office visits appear to independently decrease the placebo response.
authors.affiliation: Division of Gastroenterology, Department of Medicine, Beth Israel Deaconess Medical Center, Boston, MA 02215, USA.
authors.forename: S M
authors.initials: SM
authors.lastname: Patel
authors.forename: W B
authors.initials: WB
authors.lastname: Stason
authors.forename: A
authors.initials: A
authors.lastname: Legedza
authors.forename: S M
authors.initials: SM
authors.lastname: Ock
authors.forename: T J
authors.initials: TJ
authors.lastname: Kaptchuk
authors.forename: L
authors.initials: L
authors.lastname: Conboy
authors.forename: K
authors.initials: K
authors.lastname: Canenguez
authors.forename: J K
authors.initials: JK
authors.lastname: Park
authors.forename: E
authors.initials: E
authors.lastname: Kelly
authors.forename: E
authors.initials: E
authors.lastname: Jacobson
authors.forename: C E
authors.initials: CE
authors.lastname: Kerr
authors.forename: A J
authors.initials: AJ
authors.lastname: Lembo
mesh_terms: D002986:Clinical Trials as Topic; D006801:Humans; D043183:Irritable Bowel Syndrome / Q000188:drug therapy*; D015990:Placebo Effect; D011153:Population; D016032:Randomized Controlled Trials as Topic; D012107:Research Design