Research is pretty clear that a placebo can enhance interventions while nocebo can be detrimental. Even in the world of sports, an athlete's performance can be affected by placebo and nocebo. What is unknown, is the magnitude of the influence. Past reviews have only assessed nutritional aids while this review took a broader approach.
Small to moderate effect sizes were found for nutritional and mechanical ergogenic aids. Larger placebo effects were found when participants were led to believe they were given banned performance-enhancing ergogenic aids and Transcutaneous Nerve Stimulation. Moderate effects were found in studies investigating placebo effects of caffeine, amino acids, and modified tennis rackets.
The magnitude of the placebo effect varies depending on the type of placebo and on how the effects are induced. Pre-conditioning procedures lead to larger placebo effects than positive and negative expectations This is consistent with meta-analyses in other research domains.
Belief effects are powerful. We need controlled research to better differentiate mechanisms of effect.