During the clinical evaluation of drugs for various diseases, it has been found that drugs known to be inactive still produce what appears to be a healing response.   This presents a complication in the design of clinical drug trials since their purpose of is to determine the specific contribution of an active drug to the healing process.  To isolate the efficacy of the drug alone, drug trials are usually designed to compare the test drug to that of a physically identical, yet inert substance.  Such inert substances are known as placebos.  

In clinical drug trials, the response of the trial drug is often tested against the inactive placebo.  as the difference between The placebo effect is the measurable or observable effect on a person or group that has been given a placebo rather than an active substance.  It is likely that much of the response to homeopathic medicines is attributable to the placebo effect.  

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