The present invention generally relates to the use of tracers, for example, to track a substance that has been administered to a subject. A tracer may be separate from the substance to be administered, or in some cases, the tracer and the substance are chemically bonded to each other. When administered to a subject (e.g., ingested), the tracer and the substance to be administered may be absorbed and/or metabolized by the subject, and the pharmacokinetic activity of the tracer within the subject may be related to the pharmacokinetic activity of the substance. Thus, by following the pharmacokinetic activity of the tracer, the pharmacokinetic activity of the substance may be determined. The tracer itself may be determined within a subject using any suitable method, depending on the tracer. As an example, the tracer may be determined by administering an indicator able to interact with the tracer. For instance, the indicator may be applied to the skin of the subject, and the indicator may give a different visual appearance based on the concentration of the tracer, or otherwise exhibit a determinable change in a property of the indicator. Other aspects of the invention are generally directed to methods of making or using tracers, methods of promoting the making or use of such tracers, kits involving such tracers, and the like.
