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Evidence-Based Dermatology: Research Commentary
December 2001

The Placebo Effect: And Another One Bites the Dust

Author Affiliations

DamianoAbeniMD, MPHMichaelBigbyMDPaoloPasquiniMD, MPHMoysesSzkloMD, MPH, DrPHHywelWilliamsMD

Arch Dermatol. 2001;137(12):1639-1640. doi:10.1001/archderm.137.12.1639

Is the Placebo Powerless? An Analysis of Clinical Trials Comparing Placebo With No TreatmentHrobjartsson A, Gotzsche PCN Engl J Med. 2000;344:1594-1602

Placebo treatments have been reported to help patients with many diseases, but the quality of the evidence supporting this finding has not been rigorously evaluated. The objective of Hrobjartsson and Gotzsche's study was to conduct a meta-analysis of clinical trials in which patients were randomly assigned to either treatment with placebo or no treatment. A placebo could be pharmacological (eg, a tablet), physical (eg, a manipulation), or psychological (eg, a conversation). They systematically searched MEDLINE, EMBASE, PsychLIT, Biological Abstracts, and the Cochrane Controlled Trials Register for trials published before the end of 1998. They included only trials with concealed random assignment, masked outcome assessment, and dropout rates of less than 50%. For each trial with binary outcomes (eg, stroke or no stroke), they calculated the relative risk of an unwanted outcome. The relative risk was defined as the ratio of the number of patients with an unwanted outcome to the total number of patients in the placebo group divided by the same ratio in the untreated group. A relative risk below 1.0 indicated a beneficial effect of placebo. For trials with continuous outcomes (eg, a change in diastolic blood pressure), they calculated the standardized mean difference, defined as the difference between the mean value for an unwanted outcome in the placebo group and the corresponding mean value in the untreated group divided by the pooled SD. A value of −1 signified that the mean in the placebo group was 1 SD below the mean in the untreated group, indicating a beneficial effect of placebo. The pooled relative risk of an unwanted outcome for trials with binary outcomes and the pooled standardized mean difference for trials with continuous outcomes were calculated using random effects calculations. Hrobjartsson and Gotzsche performed preplanned sensitivity analyses to study the effects of the type of placebo used, the type of outcome, and the effect of sample size on their results.

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