Background
The placebo response of sham acupuncture in patients with primary dysmenorrhea is a substantial factor associated with analgesia. However, the magnitude of the placebo response is unclear.

Objective
This meta-analysis assessed the effects of sham acupuncture in patients with primary dysmenorrhea and the factors contributing to these effects.

Search strategy
PubMed, Embase, Web of Science, and Cochrane CENTRAL databases were searched from inception up to August 20, 2022.

Inclusion criteria
Randomized controlled trials (RCTs) using sham acupuncture as a control for female patients of reproductive age with primary dysmenorrhea were included.

Data extraction and analysis
Pain intensity, retrospective symptom scale, and health-related quality of life were outcome measures used in these trials. Placebo response was defined as the change in the outcome of interest from baseline to endpoint. We used standardized mean difference (SMD) to estimate the effect size of the placebo response.

Results
Thirteen RCTs were included. The pooled placebo response size for pain intensity was the largest (SMD = ?0.99; 95% confidence interval [CI], ?1.31 to ?0.68), followed by the retrospective symptom scale (Total frequency rating score: SMD = ?0.20; 95% CI, ?0.80 to ?0.39. Average severity score: SMD = ?0.35; 95% CI, ?0.90 to ?0.20) and physical component of SF-36 (SMD = 0.27; 95% CI, ?0.17 to 0.72). Studies using blunt-tip needles, single-center trials, studies with a low risk of bias, studies in which patients had a longer disease course, studies in which clinicians had < 5 years of experience, and trials conducted outside Asia were more likely to have a lower placebo response.

Conclusion
Strong placebo response and some relative factors were found in patients with primary dysmenorrhea.

PROSPERO registration number: CRD42022304215.

Key words: Primary dysmenorrhea, Pain, Meta-analysis, Sham acupuncture, Placebo response