A placebo is an inactive treatment frequently used in medical research. The classic example of a placebo is a pill given to a patient who thinks the pill has active ingredients to treat a particular condition or symptom when in fact the ingredients are inert (without any active ingredients.)
When a patient responds positively to a placebo is known as the "placebo effect." The placebo effect can be produced by inert tablets, by sham surgery, and by false information.
The magnitude of the placebo effect can vary widely from study to study. Studies that measure subjective attitudes (such as pain relief) usually have a greater placebo effect than trials in which evidence is independent of subjective attitudes.
The reasons placebos sometimes work are psychological. Some experts argue that people so strongly associate medication with healing that simply taking a placebo can make a patient feel better. Others claim positive expectations trigger the release of body chemicals that act as natural pain relievers. The therapeutic relationship between physicians and patients also may play a role.