We model the pathways of nandrolone in the body, an anabolic steroid widely used as a performance enhancing drug (PED). The model generates the dynamics of nandrolone and its metabolites. PED tests check for the presence of a primary metabolite of nandrolone, 19-NA in urine. To cheat in these tests, PED users typically use inhibitors that reduce the urinary concentration of 19-NA. One such inhibitor is finasteride. Finasteride’s main effect in the body is the inhibition of reductase enzymes that turn nandrolone into its metabolite 19-NA. To capture this effect, we include structures for finasteride and reductase enzymes in the model. The model is tested by fundamental structure validity tests. We also show that the model behavior is consistent with experimental data in the literature. We finally investigate the potential ways by which the drug users may cheat in PED tests and make suggestions for improved testing as counter-measures.@en