A placebo is a substance that has no significant therapeutic effect.

Placebos are often given as a way of creating a control group due to the existence of "the placebo effect," the positive influence that comes simply from the expectation that a medication or treatment will be effective. In order to rule out the possibility that an experimental treatment is effective due only to the placebo effect, the control group will take a placebo and their results will be compared to the results of experimental group.