A blinded (or masked) clinical trial is a field study of a drug in which the recipient does not know if he is receiving the actual drug versus a placebo. A double-blind clinical trial is one in which both the recipient and the administrator does not know if the recipient is receiving the actual drug.

The blinded trial approach has been shown to provide a more complete understanding of a drug's effectiveness, benefits, and the range of possible adverse reactions without the bias of the "placebo effect". It has been confirmed that a person's beliefs and hopes about a treatment, combined with their suggestibility, may have a significant biochemical effect.

Placebo effect

The placebo effect is the measure or observed effect on a person or group that has been given a placebo treatment.

A placebo is an inert substance, or "fake" surgery or therapy, used as a control in an experiment or given to a patient for its possible or probable beneficial effect. Why an inert substance, a so-called "sugar pill," or a fake surgery or therapy would be effective, is not yet completely known.

Many believe the placebo effect is psychological, due to either a real effect caused by belief or to a subjective delusion. For example: "If I believe the pill will help, it will help." Often, the patient's physical condition does not change, but it feels like it has. For example, Irving Kirsch, a psychologist at the University of Connecticut, believes that the effectiveness of Prozac and similar drugs may be attributed almost entirely to the placebo effect.