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 ( ). Conclusion. The placebo effect of acupuncture showed a unique pattern, which was affected by insufficient patient blinding. 1. Introduction Randomized trials are necessary to evaluate the effectiveness of acupuncture because acupuncture provides inconsistent benefits to patients who are randomly allocated to acupuncture treatment compared to those who directly choose [1]. Randomized trials can provide unbiased results about the efficacy of acupuncture if acupuncture is controlled with a sham acupuncture. For this aim, many efforts have been made to develop optimal sham acupuncture. In fact, sham acupunctures have changed from normal needling of nonacupoints to minimal needling of nonacupoints and nonpenetrating stimulation [2]. Nevertheless, it has been reported that all types of sham acupuncture may not be physiologically inert [3–5]. Moreover, according to a series of recent studies [6, 7], there is a high possibility of publication bias, where trials with negative results in a treatment group would be less likely to be published, in the three-armed randomized trials including sham and no-treatment groups. When the bias was considered, the magnitude of the placebo effect was calculated to be large [6]. However, all these findings [3–7] should be interpreted with caution because they were based on the data measured at one time point, as specified by authors of trials. To date, it has not been studied how the placebo