Pethidine (meperidine), a synthetic opiate, formally used as an analgesic in surgery and obstetrics, has been an abused drug of choice for some doctors. A case is presented in which a doctor, who previously admitted to using pethidine, was suspected of re-using, following a second positive urine test. A laboratory had reported the presence of pethidine in the doctor's urine; however, the doctor denied re-use. The norpethidine (normeperidine) metabolite, normally found in urine, had not been detected, raising concern over the laboratory's conclusion and necessitating an independent investigation. Because the major metabolite of pethidine is pethidinic acid (meperidinic acid), accounting for approximately 40% of the excreted dose, its presence or absence were deemed to be important criteria in interpreting the laboratory result. Pethidinic acid was synthesized by alkaline hydrolysis of pethidine and used as a control. Urine samples from a patient receiving pethidine for pain, from the previous pethidine use of the doctor, and the urine under question plus the control were analyzed for the presence of pethidinic acid using electrospray mass spectrometry. Pethidinic acid was found in all samples except the one under dispute. The absence of pethidinic acid appeared to corroborate the doctor's denial of re-use.