Medical Definition of active placebo
: a substance used in controlled experiments (such as a double-blind study of a drug) that has no therapeutic effect on the condition being treated but may produce side effects (such as drowsiness or nausea) similar to those of the substance whose effectiveness is being tested <Unlike previous studies, the present trial used an active placebo (low-dose lorazepam) that mimics the adverse effects of the active treatments without producing pharmacologic analgesia.—Ian Gilron et al., The New England Journal of Medicine, 31 Mar. 2005>
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