Background <p>The back-calculation of concentrations of narcotic and psychotropic substances (NPS) detected in blood to the forensically relevant time of the offence is based on their elimination kinetics. Due to the scarcity of literature data and ethical constraints on conducting volunteer studies, the present study aimed to derive forensically applicable elimination kinetics for methamphetamine (MA) and tetrahydrocannabinol (THC) from the results of retrospective blood serum analyses conducted in connection with road traffic offences using an alternative, nonexperimental approach.</p> Methods <p>All NPS findings from the Institutes of Legal Medicine in Dresden (2003–2020) and Leipzig (2011–2020) were included in the analysis. From the positive MA and THC cases those were selected that reported results from two blood samples collected within 48 h. After scientific plausibility assessment, inclusion criteria were defined for the zero-order elimination rate (k₀) and the first-order elimination rate constant (kₑ). Pharmacokinetic calculations for zero-order and first-order kinetics were performed under the assumption that the time of the first blood sample corresponded to t₀ and that of the second blood sample to t₁.</p> Results <p>A&#xa0;simplified estimation based on zero-order kinetics yielded an elimination rate for MA for forensic purposes of approximately 9 ng/mL/h. Under first-order kinetics, the elimination rate constant was approximately 0.05 h⁻¹, corresponding to a&#xa0;mean half-life of 16 h. For THC highly variable results were obtained that did not enable reliable back-calculation for forensic purposes using the applied alternative approach.</p> Discussion <p>The alternative approach used in this study to estimate hourly elimination rates appears to be applicable for MA but is not suitable for THC. The heterogeneous results for THC are mainly attributable to differences in elimination kinetics between occasional and chronic cannabis users.</p>