Abstract: The present invention relates to a method, composition and a kit for verifying the source of biological samples when collected for testing, e.g., for the presence of drugs of abuse. The method generally involves providing a subject with a composition comprising at least one detectable consumable marker and optionally at least one metabolizable detectable consumable marker; obtaining a biological sample from the subject after consumption of the composition; and detecting the presence or absence of the detectable consumable marker in the sample; wherein the presence of the detectable marker indicates that the subject is the source of the sample, and wherein the absence of the detectable marker indicates that the subject is not the source of the biological sample.