Several compounds that inhibit cytochrome P4501 (CYP1) enzymes have been shown to actas aryl hydrocarbon receptor (AHR) agonists by reducing the metabolic turnover of the endogenous receptor ligand 6-formylindolo[3,2-b]carbazole (FICZ). In this study we aimed at investigating whether a group of widely prescribed drugs, namely ketoconazole (KTZ),omeprazole (OME) and primaquine (PQ), can act as indirect AHR activators via this mechanism. Inhibitory effects of KTZ, PQ, and OME were measured in CYP1A1 expressing supersomes and all three drugs inhibited CYP1A1 activity. KTZ was the most efficient inhibitor and HPLC analysis revealed that KTZ slowed down the metabolic turnover of intracellular FICZ. All three drugs induced the catalytic activity of CYP1A1 (7-ethoxyresorufin-O-deethylase, EROD) in HaCaT cells as well as increased the expression of CYP1A1 mRNA. Co-exposure to the drugs with FICZ prolonged and enhanced FICZ-induced AHR activation in a synergistic manner. Our findings indicate that KTZ activate AHR by inhibiting the metabolic turnover of FICZ. Interestingly, PQ and OME seem to act by other mechanisms that sensitize the cells to FICZ-dependent transcriptional activation of the AHR.To the author’s knowledge, this is the first publication indicating that KTZ, OME, and PQ can superinduce AHR signaling by increasing the responses to an endogenous receptor ligand.