The detection of putative 18-methyl-19-nortestosterone metabolite in a forensic bodybuilder's urine sample collected as part of a criminal proceeding has triggered a follow up investigation. Four different dietary supplements in the possession of the suspect were examined with regard to possible precursor steroids. This led to the detection of the declared ingredient methoxydienone, which was confirmed by both, GC-MSMS and LC-HRMSMS. As neither 18-methyl-testosterone, nor 18-methyl-19-nortestosterone were detectable in the supplements, the possibility that the metabolite originates from methoxydienone was investigated. For this purpose, the metabolic fate of methoxydienone was studied in vitro using human HepG2 cells as well as in vivo by a single oral administration. While the 18-methyl-19-nortestosterone metabolite was not generated by HepG2 cells incubated with methoxydienone, it was observed in the urine samples collected at two, six, ten and 24 hours after methoxydienone administration. Moreover, the potential binding of methoxydienone as ligand to the human androgen receptor was modelled in silico in comparison to 18-methylnandrolone, for which androgen receptor activation had been shown in an in vitro approach before. In conclusion, we could ascribe the presence of the 18-methyl-19-nortestosterone metabolite in a forensic urine sample to originate from methoxydienone present in dietary supplements. Methoxydienone was observed to be slowly degrade by demethylation of the methoxy substituent in liquid solutions. While no compound-specific intermediates were identified that allowed differentiation from other 18-methylsteroids, the 18-methyl-19-nortestosterone metabolite proved to be a suitable marker for reliable detection in doping analysis.