The purpose of this study was to analyze the composition of 103 dietary supplements bought on the internet. The supplements were dispatched in four different categories according to their announced contents [creatine, prohormones, "mental enhancers" and branched chain amino acids (BCAA)]. All the supplements were screened for the presence of stimulants and main anabolic steroids parent compounds. At the same time, the research was focused on the precursors and metabolites of testosterone and nandrolone. The study pointed out three products containing an anabolic steroid, metandienone, in a very high amount. The ingestion of such products induced a high quantity of metandienone metabolites in urines that would be considered as a positive antidoping test. The results have also shown that one creatine product and three "mental enhancers" contained traces of hormones or prohormones not claimed on the labels and 14 prohormone products contained substances other than those indicated by the manufacturer. The oral intake of the creatine product revealed the presence of the two main nandrolone metabolites (19-norandrosterone and 19-noretiocholanolone) in urine.