The lower limits of detection of the analytical techniques currently used for drug testing in horses result in the dilemma of whether or not to report trace levels of drugs legitimately used for therapeutic medication. A non-experimental pharmacokinetic/pharmacodynamic approach for the determination of irrelevant drug plasma concentrations (IPC) and irrelevant urine concentrations (IUC) has been put forward by Toutain and Lassourd (2002). The published plasma clearance is used to transform an effective dose into an effective concentration (EPC). EPC is then transformed into an IPC by applying a safety factor (SF). This method is based on several assumptions ( eg drug effects reversibly driven by plasma concentration, linearity of drug disposition). To explore the feasibility and attractiveness of this approach, this article reports EPC, IPC and IUC for a selection of drugs.