ABSTRACT

Background:
A body of research has shown that deceptively administering a placebo described as caffeine can significantly improve sport performance. We aimed to synthesize and estimate the effect size of the deceptive administration of a placebo described as caffeine on sport performance outcomes (e.g. bench press, cycling time-trial, repeated sprint).

Methods:
We searched for studies sampling healthy adults from database creation to July 2026. The standardized mean difference (d) between placebo caffeine and baseline was calculated and Cochrane's risk of bias assessment was used.

Results:
Seventeen studies were included. Nine studies were judged to be of high-risk of bias and eight unclear. Random-effect models revealed a small-to-moderate effect of placebo caffeine benefiting performance (d = 0.49, 95% CI = 0.28 to 0.69) and meta-regressions indicated that pooled effects were more likely to be greater when researchers were blinded (b = 0.52 ± 0.24, 95% CI = 0.01 to 0.99) and in studies sampling more males (b = -0.93 ± 0.38, 95% CI = -1.68 to -0.19).

Discussion:
Data highlight that placebo effects of caffeine can significantly affect sport performance with a pooled effect estimate suggesting a small-to-moderate effect. However, as per the GRADE assessment, the overall quality of evidence was low. As such, given the high risk of bias across all studies and the variation in confidence interval, stronger research designs are needed to examine placebo effects of caffeine, whereby researchers are blinded to treatment administration and allocation concealment. No funding was received for the review and the protocol can be found here: doi: 10.17605/OSF.IO/W8KY6.

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