The concentration of an administered compound, such as a drug (D), in an organ or a bodily fluid, such as blood, is determined directly through detecting the drug (D) or its metabolites (DM) in sweat. The concentration may be determined indirectly by administering the drug (D) together with one or more tracer compounds (T, T2) or metabolites thereof (TM, T2M) or by detecting concentrations and trends of other analytes present in the body that react to the presence of the drug (D). By determining tracer concentration in sweat, the concentration of the drug (D) in blood or an organ can be determined. The tracer (T, T2) is a compound selected for ease of detection in sweat, known metabolic and solubility profiles that correspond to those of the drug (D), and safety of use. A smart transdermal delivery patch (300) is used to administer a dosage of drug to a wearer in coordination with at least one sweat sensor (324) reading conveying information about the wearer.
