placebo (pləsē´bō), inert substance given instead of a potent drug. Placebo medications are sometimes prescribed when a drug is not really needed or when one would not be appropriate because they make patients feel well taken care of. Placebos are also used as controls in scientific studies on the effectiveness of drugs. So-called double blind experiments, where neither the doctor nor the patient knows whether the given medication is the experimental drug or the placebo, are often done to assure unbiased, statistically reliable results. A traditional placebo's lack of side effects, however, often identifies it, so an older drug is sometimes used in drug tests instead of or in addition to a placebo.
The "placebo effect" is an apparent improvement in health due not to any treatment but only to the patient's belief that he or she will improve (as by taking a dummy pill that is thought to be a cure). A report released in 2001, however, reviewed 114 studies where use of a placebo was compared to both treatment and no treatment and found no placebo effect with respect to measurable medical conditions, such as blood pressure and cholesterol levels. An opposite, or "negative placebo effect," has been observed when patients believe their health will get worse.