A method for use in detecting and monitoring effects experienced by a subject taking a first drug, drug A, and at least one other substance, substance B, the method comprising: (a) obtaining a plurality of measured values of a physiological characteristic of the subject over a time period; (b) comparing the measured values to a predefined signature, the drug A signature, associated with the first drug, and thereby calculating a measure, Adiff, of the difference between the measured values and the drug A signature; (c) comparing the measured values to a predefined signature, the substance B signature, associated with the at least one other substance, and thereby calculating a measure, Bdiff, of the difference between the measured values and the substance B signature; (d) comparing the measured values to a predefined signature, the DO signature, associated with a desired physiological state of the subject, and thereby calculating a measure, DOdiff, of the difference between the measured values and the DO signature; and (e) combining Adiff and DOdiff to produce an output.