A pill or preparation with no medicinal value, given either to please a patient or to help test a given therapy. Latin for “I shall be pleasing,” a placebo is traditionally an inactive pill that a doctor prescribes to oblige a patient. Doctors have known for many years that people receiving a placebo tend to feel better even though the pill contains no medicine. This phenomenon has come to be known as “the placebo effect.”

Researchers have to take the placebo effect into consideration when testing drugs or other treatments because study participants may tend to feel better whether or not the treatment achieves its goal. To compensate for this effect and “level the playing field,” treatments are often tested in placebo trials: Volunteers may get either the real treatment or a placebo, and usually neither the doctor nor the patient knows which is which (a so-called “double-blind” trial). The researchers can then compare the apparent benefits of the treatment and the placebo to see whether the treatment produced any substantial difference.