From Longman Dictionary of Contemporary Englishplacebo effectplacebo effectwhen someone feels better after taking a placebo, even though it has not had any effect on their body → placebo
Examples from the Corpus
placebo effect• However, you then have another problem called the placebo effect.• With diseases such as rheumatoid arthritis, it is less obvious how the placebo effect works.• Medicine has generally regarded the placebo effect as a nuisance: it does make research on new medical drugs very difficult. 