blinded study

Clinical trials of drugs are often done blinded so that the patient does not know (is blinded as to) whether they are receiving the product being tested or the control/placebo to ensure that the results of a study are not affected by a possible placebo effect (by the power of suggestion).

A type of study in which the patients (single-blinded) or the patients and their doctors (double-blinded) do not know which drug or treatment is being given. The opposite of a blinded study is an open label study.