Among 22 studies of which SMD and SD were available, two studies had two placebo control groups, leaving 24 units of data for a meta-analysis. No study from MI category provided adequate data for pooling. The meta-analysis of studies in DD category showed acupuncture produced better clinical outcomes compared to placebo acupuncture (SMD −0.43, 95% CI −0.68 to − 0.18, p = 0.0006; Fig. 3), whereas the studies of FD category found no significant difference between real and placebo groups (SMD −0.12, 95% CI −0.26 to 0.01, p = 0.08; Fig. 3). When two categories were assigned as subgroups, a test for subgroup differences also showed that pooled effect for real acupuncture from DD category is greater than that of FD category (χ2 = 4.54, p = 0.03, I2 = 78.0%; Fig. 3).Fig. 3Clinical outcomes by PIL categories. Anonymized studies are from (a) FD category, and (b) DD category, respectively. A test for subgroup differences indicates that studies from DD category report greater effects of real acupuncture compared to those from FD category. CI, confidence intervals; DD, deceptive disclosure of placebo acupuncture; FD, full disclosure of placebo acupuncture; PIL, participant information leaflet; SE, standard error; Std., standardized