The supply of recreational drugs has changed in the early 2000s and users increasingly buy cathinones over the Internet. There is a potential for significant toxicity associated with their use. Cathinone named “NRG-3” seems to be the most popular in Parisian CEIP data: the adverse events related to use of NRG-3 from 2011 to 2015 are reported. We aim at determining the chemical composition of NRG-3 available over the Internet and whether products differ depending on retailers. Three powders labelled “NRG-3” were purchased on July 2012 from three different Internet sites, moreover one capsule and one powder labelled “NRG-3” was given by drug users. These were analysed by LC–MSMS to determine active ingredients. The cathinones class was detected, without active adulterant. A great inconsistency in the qualitative and quantitative composition of products bearing identical labelling “NRG-3” was shown. We found two different single components: methylene dioxypyrovalerone (MDPV) and α-pyrrolidinopentiophenone (α-PVP) with mixture preparations in (3/5) samples. α-PVP varied from 4 to 80% and MDPV varied from 0 to 70%. There was significant variation of the products contained in NRG-3. This variation could be of clinical significance as the NRG-3 can be associated with worrisome toxicity. Marked agitation with violent and unpredictable acts is reported with MDPV: this molecule has a higher ability to cross the blood brain barrier because the pyrrolidine ring confers a low polarity. Given the plethora of NPS, proper drug identification is core essential for harm reduction associated with empowered users through education.