A placebo is a medicine or diet which may have a medical effect based solely on the power of suggestion , an effect known as the placebo effect or placebo response . The placebo effect has sometimes been defined as a physiological effect caused by the placebo , but Moerman and Jonas have pointed out that this seems illogical , as a placebo is an inert substance which does not directly cause anything . Instead they introduced the word `` meaning response '' for the meaning the brain associates with the placebo , which causes a physiological placebo effect .