Placebo effects of caffeine on sport performance: a systematic review and meta-analysis.
Philip Hurst, Janina Hanssen, Felipe Marticorena et al.
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In 30 seconds
This systematic review and meta-analysis examined whether deceptively describing a placebo as caffeine affects sport performance in healthy adults. Across 17 studies, the pooled effect favoured placebo caffeine, with a small-to-moderate standardized mean difference (d = 0.49, 95% CI = 0.28 to 0.69) compared with baseline. Meta-regression found larger pooled effects were associated with researcher blinding and with studies sampling more males. However, all studies had high or unclear risk of bias, and the GRADE assessment rated the overall evidence as low quality.
Key findings
- Across 17 studies, placebo caffeine was associated with improved performance versus baseline (d = 0.49, 95% CI = 0.28 to 0.69).
- Nine studies were judged at high risk of bias and eight at unclear risk; overall evidence quality was low.
- Pooled effects were more likely to be greater when researchers were blinded (b = 0.52 ± 0.24, 95% CI = 0.01 to 0.99).
- Pooled effects were more likely to be greater in studies sampling more males (b = -0.93 ± 0.38, 95% CI = -1.68 to -0.19).
Why it matters
The findings suggest that expectations created by describing a placebo as caffeine may contribute to measured sport performance, so performance outcomes may reflect more than the physiological effects of caffeine itself. The low-quality evidence and widespread risk of bias mean the pooled estimate should be interpreted cautiously; better-controlled studies are needed to clarify the size and reliability of this effect.
What to keep in mind
The review reports high or unclear risk of bias across all included studies and low overall evidence quality. The authors also note variation in confidence intervals and call for stronger research designs.