1. a pharmacologically inert substance, such as a sugar pill, that is often administered as a control in testing new drugs. Placebos used in double-blind trials may be dummies or active placebos. Formerly, placebos were occasionally used as diagnostic or psychotherapeutic agents, for example, in relieving pain or inducing sleep by suggestion, but the ethical implications of deceiving patients in such fashion makes this practice problematic.

2. any medical or psychological intervention or treatment that is believed to be “inert,” thus making it valuable as a control condition against which to compare the intervention or treatment of interest. See placebo effect.