A system is disclosed for monitoring a property of an ingested or in-taken drug, food, drink or toxic substance, non-invasively or minimally invasively, which can also identify the subject person being monitored, if desired. The system comprises: a means of labeling the substance with a labeling media to have a useful signature indicative of, or bearing a relation to the property; a means to allow the signature to be read non-invasively or minimally invasively; and a means to identify, in any manner, who is being monitored.