A placebo is a substance or treatment with no active therapeutic effect. Common placebos include inert tablets (like sugar pills), inert injections (like saline).
A placebo do not contain an active ingredient meant to produce any pharmacological effect.
Researchers use placebos during studies to help them understand what effect a new drug or some other treatment might have on a particular condition.
For instance, some people in a study might be given a new drug to lower high blood pressure. Others would get a placebo. None of the people in the study will know if they got the real treatment or the placebo.
Researchers then compare the effects of the drug and the placebo on the people in the study. That way, they can determine the effectiveness of the new drug and check for side effects.