An inert substance, sometimes called a “sugar pill” (although it isn’t necessarily made from sugar). Placebos are often used in randomized trials to test an intervention. For example, if researchers want to test whether drug × reduces the risk of catching a cold, they can randomly assign participants to either a group who will take drug × or a group who will take an identical-looking but ineffective placebo. At the end of the study, the researchers compare how often people in the two groups caught colds.

The purpose of a placebo is to help ensure that patients in each study group are treated in exactly the same way. Without a placebo group, everyone would know which patients were getting the drug under investigation. Those patients might be treated differently—or might report their information differently—and this could bias the results.