PurposeLisdexamfetamine (LDX), which is used for the treatment of attention-deficit/hyperactivity disorder and narcolepsy, is composed ofl-lysine attached to dextroamphetamine (d-amphetamine). In this article, we report a forensic autopsy case in which prescription drugs were unknown at autopsy. While amphetamine was detected, methamphetamine could not be detected by liquid chromatography–tandem mass spectrometry (LC–MS/MS) in any of samples collected. Thus, we aimed to quantify LDX concentrations in autopsy samples and to prove that the amphetamine detected in this case was due to metabolized LDX.MethodsFemoral vein blood, cardiac whole blood, urine, and gastric content samples were taken at autopsy for toxicological analysis. Qualitative and quantitative analyses were performed using LC–MS/MS. In addition, optical isomer separation for the amphetamine detected was conducted. The stability of LDX in whole blood and urine was also examined at three different temperatures.ResultsThe concentrations of LDX were < 4.00, 30.9, and 4.42 ng/mL in whole blood, urine, and gastric content samples, respectively. The concentrations of amphetamine were 329, 510, 2970, and 915 ng/mL in femoral vein blood, heart whole blood, urine, and gastric contents, respectively. The amphetamine detected in this case was identified to be onlyd-amphetamine by optical isomer separation. Thed-amphetamine detected was considered to be derived from LDX. Stability experiments revealed that LDX in whole blood decreased at ambient temperature.ConclusionsThe results in the present case report may be useful in interpreting whether or not the amphetamine detected in a cadaver is a metabolite of LDX.Supplementary InformationThe online version contains supplementary material available at 10.1007/s11419-022-00654-6.