Objectives: This study aimed to identify the factors that might have contributed to the conflicting outcomes about the efficacy of acupuncture for tension-type headache (TTH) through systematically reviewing relevant randomized controlled trials.

Methods: Thirteen (13) databases were searched from their inception until August 2010. There were no restrictions on language or year of publication. Included studies were randomized controlled trials comparing real with sham acupuncture, with patient selection guided by the International Headache Classification, and reported headache days. Meta-analyses and subgroup analyses were undertaken to compare the effects of real and sham acupuncture interventions and the effects of acupuncture with various needling techniques and treatment modes.

Results: Forty-three (43) studies were retrieved for further assessment from 120 potential studies. Finally, five studies of high methodological quality were included in this review. Standard mean difference (SMD) of the included studies showed no statistical significance between real and sham acupuncture (-0.31; 95% confidence interval [CI] -0.72 to 0.09), however, the heterogeneity among the studies was high (I(2)=81%). Subgroup analyses reduced heterogeneity, and showed that electro-acupuncture (SMD-1.60; 95% CI -2.33 to -0.88) to be more efficacious than manual acupuncture (SMD -0.13; 95% CI -0.41 to 0.14); needle retention with 30 minutes (SMD-0.46; 95% CI -0.87 to -0.06) being better than no needle retention (SMD 0.45; 95% CI -0.11, 1.01); and twice-a-week treatment (SMD -0.46; 95% CI -0.87 to -0.06) was better than once-a-week treatment (SMD 0.45; 95% CI -0.11, 1.01).

Conclusions: Acupuncture stimulation mode, needle retention, and treatment frequency could be important factors contributing to the outcome of acupuncture for TTH. Further studies are warranted to determine treatment parameters to ensure effective translation of RCTs outcomes of acupuncture for patients with TTH.