Prohibited doping substances in sports are usually detected by the excretion of the mother compounds and their metabolites. My goal is to further improve the analytical confidence in identification of these substances especially of highly similar, often diastereomeric structures after phase-I-metabolism. Therefore, I am focusing on metandienone, which is a frequently used anabolic androgenic steroid and my instrumental approach is mainly GC-EI-MS. But chemical synthesis of reference material and subsequent structure confirmation by GC-QTOF-MS and NMR are part of my used methods as well. In the end, new insights as a deeper understanding of the fragmentation of the per-TMS derivatives of metabolite structures and their inevitable chromatographic separation will be applied on a controlled excretion study in humans analyzed by GC-QQQ-MS.