E-vapor products (EVPs) consumption has steadily increased worldwide over the past decade. Ever since their introduction, there have been discussions about the potential health risks of EVPs in the scientific and public health community. These discussions are complicated by contrary findings, especially with respect to aldehydes, which may be formed in the aerosol generated from EVPs. In order to assess the internal dose of the major ingredients in EVP users and to investigate their metabolism and potential decomposition products, we conducted a clinical study with 20 EVP users and 5 smokers of conventional non-filter cigarettes. We used stable-isotope labelled e-liquid constituents and mass spectrometry based detection in various body fluids (plasma, urine, saliva, sputum). We developed and modified several bioanalytical methods regarding the quantification of the labelled main ingredients of e-liquids, namely propylene glycol (PG), glycerol (G) and nicotine (Nic) as well as their metabolites and potential degradation products which may be formed from PG and G under pyrolysis conditions. No interferences due to the diet or exposure to other consumer products were observed for labelled PG, G, and Nic in plasma and urine. Stable-isotope labelled mercapturic acids (MA) formed from acrolein (3-HPMA) and propylene oxide (2-HPMA) were not detectable in urine of EVP users. In contrast, labelled 3-HPMA and 2-HPMA were observed in the smokers smoking non-filter cigarettes spiked with stable-isotope labelled PG, G, and Nic. Hence, inclusion of a smoker positive control group allowed us to reveal pyrolysis products specifically derived from PG and G. In conclusion, our data proved the applicability of the stable-isotope labelling concept to unequivocally assess EVP-specific internal dose of the major ingredients PG, G, and Nic as well as the presence of potential degradation products in the vapour and their further metabolism in the human body.