We disclose a method of using taggants to assess how and to what extent a drug in a drug composition that a user has consumed has decayed in response to storage conditions and time. The taggants may decay in response to environmental conditions which cause different drugs to lose their efficacy. These environmental conditions may include light, temperature, oxidation, moisture, and age. The taggants may be detected in biological samples, including urine and feces. By identifying the taggants, the drug composition and other information relating to the drug may be identified. Additionally, quantification of the different taggants may be used to determine whether the drug in the drug composition has been exposed to environmental conditions which may reduce its efficacy.