Time Course of Placebo Effect of Acupuncture on Pain: A Systematic Review

Objectives. Our objective was to investigate the time course of the placebo effect of acupuncture on pain and the factors affecting the placebo effect. Methods. Previously we retrieved three-armed randomized acupuncture trials including sham and no-treatment groups which were published until October 2009. We searched electronic databases again to identify additional trials from October 2009 to December 2011. After a screening of trials, fifteen three-armed acupuncture trials for pain were included in the analysis. Standardized mean differences between the sham and no-treatment groups were calculated for placebo effect. We then plotted the magnitude of the placebo effect over time. Results. The placebo effect gradually has increased for 12 weeks with a standardized mean difference of 0.74 (95% CI: 0.54 to 0.94). Then it decreased after 12 weeks as time passed. When the placebo effects were compared for factors including methodological qualities, they were not affected by all factors, except patient blinding. Trials with sufficient patient blinding showed a larger placebo effect at 8 weeks than those with insufficient patient blinding (P = 0.0009). Conclusion. The placebo effect of acupuncture showed a unique pattern, which was affected by insufficient patient blinding.