In extreme situations, reliable detection of the minimum therapeutic concentrations of psychotropic substances is important, since this allows one to provide adequate resuscitation. The group of benzodiazepine derivatives, which includes bromodihydrochlorophenylbenzodiazepine (phenazepam), is widely used in clinical practice. Along with the positive clinical effect, phenazepam has numerous side effects, capable of causing poisoning, even death. The study was aimed to develop the method for detection of the phenazepam metabolites by high-resolution HPLC–TMS suitable for achieving the aims and objectives of forensic medical expertise in case of the ultra-low urine substance concentrations. Urine of six patients (males and females aged 28–40), who were prescribed phenazepam and took the drug at minimum therapeutic concentrations on an ad hoc basis, was used during the study. Optimum conditions for the analyte chromatography after the urine sample preparation were defined with the phenazepam retention time of 7.05 ± 0.06 min; specific ions (m/z) 179, 183, 206, 242, 271, 285, 320, 348 (main) were defined for identification of phenazepam.