Among patients with irritable bowel syndrome (IBS) enrolled in clinical trials of conventional medical therapy, the placebo response rate is high. IBS patients also frequently use complementary and alternative medicine (CAM), which may act through an 'enhanced placebo effect'. The purpose of this study was to estimate the magnitude of the placebo response rate in CAM trials for IBS and to identify factors that influence this response. We performed a systematic review and meta-analysis of randomized, placebo-controlled clinical trials of CAM therapies for IBS identified from MEDLINE/EMBASE/PsychLIT databases from 1970 to 2006. Placebo and active treatment response rates for global symptom improvement were assessed. Nineteen studies met the inclusion criteria. The pooled estimate of the placebo response rate was 42.6% (95% confidence interval, 38.0-46.5%). Significant heterogeneity existed across trials (range 15.0-72.2%, P < 0.00001). Higher placebo response rates correlated with a longer duration of treatment (r = 0.455, P = 0.05) and a greater number of office visits (r = 0.633, P = 0.03). Among IBS patients in CAM trials, the placebo response rate is high. That this rate is similar in magnitude to that seen in conventional medicine trials suggests that the placebo response is independent of the type of therapy used and that it is not particularly 'enhanced' in CAM trials.

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