"Placebo" is a Latin word that literally means "I shall please". A placebo is a substance which does not cause any physiological response, such as a sugar pill. Sometimes, a doctor may give a patient a placebo to satisfy the patient's demand for a drug. It is very common for individuals given a placebo to report an improvement in symptoms. This is sometimes called the "placebo effect". Since this improvement cannot (by definition) be due to the action of the placebo, it may reflect the psychological effect of the subject's belief in the benefit of the pill.

In research studies to determine the efficacy of a new drug, some subjects may be given the experimental drug and some subjects may be given a placebo. The purpose of this procedure is to control for the placebo effect. Subjects are not told which kind of pill they have been given. Thus, if subjects given the test drug report experiencing some side effect (e.g., nausea) while subjects given the placebo do not, the experimenters would conclude that nausea is a valid side effect of the drug. However, if subjects given the placebo and subjects given the drug both report an improvement in migraines, the experimenters should conclude that the drug does not provide any better migraine relief than a placebo.