AbstractIn order to detect the misuse of testosterone (T) or boldenone (Bo) in doping control analysis, the confirmation of atypical findings employing the determination of carbon isotope ratios (CIR) is mandatory for issuing adverse analytical findings. Elevated concentrations of T (or elevated T/epitestosterone ratios) may result from confounding factors such as ethanol intake, and the presence of low urinary concentrations of Bo can originate from endogenous or urinary in situ production of small amounts of the steroid. As pharmaceutical preparations of Bo and T are generally depleted in 13C, their CIR differ significantly from the 13C‐enriched endogenous steroids. Some rare cases have been reported on pharmaceutical preparations showing 13C‐enriched isotope ratios that complicate the current application of CIR in sports drug testing. Therefore, the CIR of a subset of n = 157 T preparations and n = 39 Bo preparations seized in Switzerland and Germany between 2013 and 2018 was analyzed in order to estimate the possible impact of steroid preparations showing 13C‐enriched isotope ratios on the current approach to detect their misuse. All investigated Bo preparations showed CIR in the expected range between – 26.7 and −30.3‰. Within the T samples, 95% showed the expected values below −26‰ while six samples fall between −25 and −26‰ and one sample was indistinguishable from endogenously produced T with a CIR of −23.3‰.