A placebo ( plə-SEE-boh) is a substance or treatment which is designed to have no therapeutic value. Common placebos include inert tablets (like sugar pills), inert injections (like saline), sham surgery, and other procedures.

In general, placebos can affect how patients perceive their condition and encourage the body's chemical processes for relieving pain and a few other symptoms, but have no impact on the disease itself. Improvements that patients experience after being treated with a placebo can also be due to unrelated factors, such as regression to the mean (a statistical effect where an unusually high or low measurement is likely to be followed by a less extreme one). The use of placebos in clinical medicine raises ethical concerns, especially if they are disguised as an active treatment, as this introduces dishonesty into the doctor–patient relationship and bypasses informed consent. While it was once assumed that this deception was necessary for placebos to have any effect, there is some evidence that placebos may have subjective effects even when the patient is aware that the treatment is a placebo (known as open-label placebo).

In drug testing and medical research, a placebo can be made to resemble an active medication or therapy so that it functions as a control; this is to prevent the recipient or others from knowing (with their consent) whether a treatment is active or inactive, as expectations about efficacy can influence results. In a placebo-controlled clinical trial any change in the control group is known as the placebo response, and the difference between this and the result of no treatment is the placebo effect. Some researchers now recommend comparing the experimental treatment with an existing treatment when possible, instead of a placebo.

The idea of a placebo effect—a therapeutic outcome derived from an inert treatment—was discussed in 18th century psychology, but became more prominent in the 20th century. An influential 1955 study entitled The Powerful Placebo firmly established the idea that placebo effects were clinically important, and were a result of the brain's role in physical health. A 1997 reassessment found no evidence of any placebo effect in the source data, as the study had not accounted for regression to the mean.