Anabolic androgenic steroids (AAS) are prohibited in sports as performance enhancing drugs. To detect their misuse, among other methods, the world anti-doping agency (WADA) accredited laboratories monitor the urinary steroid profile (amount and ratio of endogenous steroids, precursors and metabolites) in urine samples collected from athletes. As steroid metabolism is very complex, it is very likely that selected non‑prohibited drugs influence steroid metabolite excretion. Non-steroidal anti-inflammatory drugs (NSAID), which are reported to influence steroid-metabolizing enzymes in vitro, are very frequently used drugs in sports. With the combination of in vitro and in vivo investigations, this project aims to investigate the influence of NSAID intake on the urinary steroid profile.