A placebo is a substance with no therapeutic or active ingredient. It is commonly used in studies as control, whose effects can be compared to those of the substance under investigation in order to determine the investigational intervention’s efficacy.

A placebo group in a study is a group of participants who are given the placebo, instead of the intervention under investigation. For example, a trial of a new supplement might give half the participants capsules containing the supplement, and give the other half capsules filled with an inactive substance. In order for a placebo group to serve as a control group, the only difference between the placebo group and the treatment group (i.e., the participants who receive the investigational treatment) should be whether they’re given the placebo or the treatment. Then, any differences in outcomes between the placebo-control group and the treatment group must be due to the treatment–in our example, to the new supplement. Thus, a placebo control allows researchers to evaluate both the safety and efficacy of the intervention. Since the placebo has no active ingredient, any changes that study participants in the placebo group have while taking the placebo are likely to be due to other factors, such as confounding variables, and not the medication being researched.

Why use a placebo, instead of just giving the control group no treatment at all? Using placebos can also ensure that results are not influenced by participants’ expectations or beliefs. People can report improvements in symptoms simply due to taking a pill or receiving a treatment–even when that pill or treatment is a placebo. This is a psychological, not physical, effect known as the placebo effect. Using a placebo group helps to avoid the placebo effect’s influence on the results, allowing the control and treatment groups to be compared and the treatment group’s outcomes to be fairly assessed.