Placebo
A treatment with no active ingredient, used as a comparison in clinical trials.
What it means
A placebo is an inactive treatment given as a comparison in a trial, so that a real drug's effect can be separated from everything else that happens to someone being treated. From the Latin, "I shall please".
The placebo effect is the genuine improvement people experience on inactive treatment. It is real and measurable — driven by expectation, the ritual of treatment, attention from a clinician, and the natural tendency of most conditions to improve on their own.
A nocebo effect is the reverse: side effects experienced from an inactive treatment, because they were expected.
In practice
Placebo-controlled trials are the reason we know statin muscle aches largely occur equally on placebo, and why many treatments that felt effective in practice turned out not to be.
Some conditions — pain, nausea, sleep, mood, irritable bowel — show particularly strong placebo responses, which is precisely why they need controlled trials most.
Why it matters
"It's the placebo effect" is not the same as "you are imagining it". The improvement is real; the question is whether the specific treatment caused it.
This matters commercially. A supplement or private treatment marketed on testimonials alone has not been separated from placebo, natural recovery and selection of the people who happened to do well.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026