|LC-MS/MS enables quantitation of drug/metabolite concentrations in urine. Carisoprodol is a skeletal muscle relaxant indicated for musculoskeletal conditions that is extensively metabolized to the active metabolite, meprobamate. The study objective was to evaluate the relationship between meprobamate and carisoprodol concentrations in urine among pain patients prescribed carisoprodol. Meprobamate and carisoprodol assays of 11,322 urine specimens from single-visit patients were done by LC-MS/MS. A weak positive correlation between log meprobamate and log carisoprodol concentrations was observed (R2=0.27). Subpopulations showing low (n=25) and high (n=10) metabolism were observed. LC-MS/MS quantitation of drug/metabolite concentrations in urine may be useful for monitoring individual metabolism.