Abstract
Objective
To determine whether trial-design, patient-type, or placebo-type factors influence
the size of the placebo analgesic effect in clinical trials.
Study Design and Setting
Trials that measured pain outcomes in Hróbjartsson and Gøtzsche's meta-analysis were
retrieved and coded for eight factors potentially predictive of placebo effect size.
Random effects meta-regression was used to explore the predictive power of each factor
on placebo effect size. The factors investigated aspects of trial design (nonstandardized
co-analgesia, co-intervention), patients (pain type, patient group, residual pain
score), and placebo (placebo type, indistinguishability, structural equivalence).
The meta-analysis undertaken in the original study was also repeated to confirm the
results.
Results
The pooled effect of placebo was 3.2 points on a 100-point scale (95% confidence interval
[CI]=1.6–4.7). None of the selected factors influenced the size of placebo effect: the
effect of all factors was close to zero, all CIs spanned 0, and P-values ranged from 0.13 to 0.90.
Conclusion
This study confirms the findings of previous researchers that, at present, the evidence
for large placebo analgesic effects in clinical trials is lacking. Importantly, this
analysis also establishes that larger placebo effects are not associated with particular
aspects of the trial methodology, patient, or placebo type.
Keywords
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Article info
Publication history
Published online: January 07, 2008
Accepted:
March 20,
2007
Identification
Copyright
© 2008 Elsevier Inc. Published by Elsevier Inc. All rights reserved.
