• Optimization of a suitable extraction protocol for alkylindoles and their metabolites from biological fluids. • Detection of alkylindoles and their metabolites by means of the GC/MS method adopted by UNODC. • In vitro incubation with human liver microsomes of alkylindoles. • Study of the metabolism of 1-butyl-3-(1-(4-methyl)naphthoyl)indole for the first time.