Data for BI calculation were available in 26 studies (Fig. 1). With one study providing three independent data sets , 28 sets of BI were calculated in total. Among the nine possible scenarios of blinding (Table 1), 50% of the studies (n = 14) were counted into blinding scenarios 1 (random guess for both acupuncture and control groups) and 5 (unblinded for acupuncture group and opposite guess for control group), which are assumed to be the ideal blinding situation. In the scenarios considered possibly problematic, which are blinding scenarios 4 (unblinded for both acupuncture and control groups) and 6 (unblinded for acupuncture group and random guess for control group), 35.7% of the studies (n = 10) were included. When the number of studies that belonged to ideal or possibly problematic blinding scenarios was compared within each category, DD/MI category had a higher rate of studies with ideal blinding than FD category (Table 1). The number of studies in S1 and S5, the ideal scenarios, was more than double the number of studies in S4 and S6 in DD/MI category (7 studies vs. 3 studies), whereas the numbers were the same in FD category (5 studies vs. 5 studies).Table 1Frequencies of nine different blinding scenarios by PIL categoriesScenarioExperimental GroupControl GroupPossible Interpretation FD % (n)DD/MI % (n)S1Random guessRandom guessPossibly most ideal from the scientific or statistical perspective11.8 (2)18.2 (2)S2Random guessOpposite guessRare5.9 (1)9.1 (1)S3Random guessUnblindedPossibly little treatment effect and no effect in control group5.9 (1)0 (0)S4UnblindedUnblindedPossibly problematic11.8 (2)9.1 (1)aS5UnblindedOpposite guessIdeal – patients tend to have wishful thinking, strong placebo effect, and any treatment administered is perceived as real treatment29.4 (5)45.5 (5)S6UnblindedRandom guessPossibly problematic – patients in control group do not know what to expect in the absence of treatment29.4 (5)18.2 (2)S7Opposite guessOpposite guessRare0 (0)0 (0)S8Opposite guessRandom guessRare5.9 (1)0 (0)S9Opposite guessUnblindedNo treatment effect at all or patients may have low expectations0 (0)0 (0)There were 17 and 11 studies in FD, and DD/MI category, respectively. aAmong the 11 studies in DD/MI category, only one article belonged to MI categoryHere, the term “Unblinded” generally means “More correct guess,” not broken blinding literallyRandom guess: −0.2 < BI < 0.2; unblinded: BI ≥ 0.2; opposite guess: BI ≤ −0.2 BI Blinding index, DD Deceptive disclosure of placebo acupuncture, FD Full disclosure of placebo acupuncture, MI Missing information on placebo acupuncture, PIL participant information leaflet