Placebo

A placebo is a treatment with no active ingredient, given as a comparison so the effect of the real treatment can be measured against it. The dummy pill, injection or procedure is matched to the real one in appearance, taste and schedule, so neither the participant nor the person judging the outcome can tell them apart. The placebo response is everything that improves in that arm: natural recovery, regression to the mean, the attention that comes with being in a study, and expectation. Only the last part is a placebo effect in the strict sense. Henry Beecher reported in 1955 that around 35% of patients respond to placebo, a figure drawn without any untreated comparison group. Hróbjartsson and Gøtzsche closed that gap in 2001 by pooling 114 trials that had both a placebo arm and a no-treatment arm. Placebo did little for objective binary outcomes and produced only a small effect on subjective ones such as pain, roughly 6.5 mm on a 100 mm scale. Expectation still counts: an openly declared placebo beat no treatment in irritable bowel syndrome, 59% versus 35% improved. Its mirror image is the nocebo effect, where warning about side effects produces them. In nutrition and longevity trials a convincing placebo is often impossible, which is why supplement results are firmer than diet results.

Last reviewed: August 25, 2026

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