Drug tasting may confound human drug discrimination studies

Unpublished observations suggested that some subjects in human drug discrimination studies acquired a drug versus placebo discrimination by tasting the capsule contents. To evaluate the prevalence of this behavior and to develop approaches to circumvent this problem, 30 normal human subjects participated in a drug discrimination study in which the study capsules (i.e., lactose and lactose + quinine) could be discriminated by tasting the capsule contents but were otherwise pharmacologically indistinguishable. Twenty percent of the subjects significantly discriminated between the capsules; this discrimination was disrupted by employing thorough mouth checks following capsule administration. Based on these findings, we recommend that human drug discrimination studies incorporate procedures that minimize the possibility of drug tasting by requiring consumption of sufficient fluid in combination with rigorous mouth checks to ensure capsule swallowing.