The present invention relates to the detection of markers in exhaled breath, wherein the detection of the presence or absence of the marker(s) in exhaled breath is used to assess various clinical data, including patient adherence in taking the medication and patient enzymatic (metabolic) competence in metabolizing the medication. An embodiment of the invention comprises a parent therapeutic agent labeled with a marker, where upon
metabolism (e.g., via enzymatic action) of the therapeutic agent, the marker becomes volatile or semi-volatile and is present in the breath. In certain related embodiments, the marker contain a
deuterium label, which is also present in the breath upon
metabolism of the therapeutic agent. In another embodiment of the invention, the therapeutic agent is associated with a
taggant (that may be either labeled or unlabeled with
deuterium), which in turn will generate a marker in the breath that is easily measurable.