This study was a laboratory simulation of the double-blind clinical study in which inactive control drugs are described as "identical matching placebos." For five of six drug categories, subjects simulating experimenters or patients significantly (P ≤.001) differentiated active drug from placebo based on physical characteristics of the medications. Thus, many of the identical matching placebos were not in fact identical but were different from the active drug in physical properties such as texture, color, and thickness.
The results suggest that the assumption that "identical matching placebos" as used in a study should be tested by preliminary comparison of the placebo with the active drug. Major recommendations are that active drug and control be administered as capsules, that research assistants be minimally aware of the experimental design of the study, that the Federal Drug Administration or National Institutes of Health formulate standard capsules for use in controlled clinical evaluation studies, and that the placebo contain active ingredients to mimic the side effects of the active drug.
Blumenthal DS, Burke R, Shapiro AK. The Validity of "Identical Matching Placebos". Arch Gen Psychiatry. 1974;31(2):214-215. doi:10.1001/archpsyc.1974.01760140066011