Here are definitions for some common terms used in clinical trials.

Placebo:
A placebo (sometimes called a “sugar pill”) is an inactive pill, liquid, or powder that has no treatment value. In clinical trials, experimental treatments are often compared with placebos to assess the treatment’s effectiveness. In some studies, the participants in the control group will receive a placebo instead of an active drug or treatment.

Control group:
A control is the standard by which experimental observations are evaluated. In many clinical trials, one group of patients will be given an experimental drug or treatment, while the control group is given either a standard treatment for the illness or a placebo.

Blinded study:
A blinded or masked study is one in which participants do not know whether they are in the experimental or control group in a research study. Those in the experimental group get the medications or treatments being tested, while those in the control group get a standard treatment or no treatment.

Double-blinded study:
A double-blind or double-masked study is one in which neither the participants nor the study staff know which participants are receiving the experimental treatment and which ones are getting either a standard treatment or a placebo. These studies are performed so neither the patients’ nor the doctors’ expectations about the experimental drug can influence the outcome.

Side effects:
Side effects are any undesired actions or effects of drug or treatment. Negative or adverse effects may include headache, nausea, hair loss, skin irritation or other physical problems. Experimental treatments must be evaluated for both immediate and long-term side effects.

For more useful definitions related to clinical trials, visit ClinicalTrials.gov Glossary.