Organic cation transporter 2 (OCT2) clears the blood of cationic drugs. Efforts to understand OCT2 selectivity as a means to predict the potential of new molecular entities (NMEs) to produce unwanted drug-drug interactions typically assess the influence of the NMEs on inhibition of transport. However, the identity of the substrate used to assess transport activity can influence the quantitative profile of inhibition. Metformin and 1-methyl-4-phenylpyridinium (MPP), in particular, display markedly different inhibitory profiles, with IC50 values for inhibition of MPP transport often being more than fivefold greater than IC50 values for the inhibition of metformin transport by the same compound, suggesting that interaction of metformin and MPP with OCT2 cannot be restricted to competition for a single binding site. Here, we determined the kinetic basis for the mutual inhibitory interaction of metformin and MPP with OCT2 expressed in Chinese hamster ovary cells. Although metformin did produce simple competitive inhibition of MPP transport, MPP was a mixed-type inhibitor of metformin transport, decreasing the maximum rate of mediated substrate transport and increasing the apparent Michaelis constant (Ktapp) for OCT2-mediated metformin transport. Furthermore, whereas the IC50 value for metformin's inhibition of MPP transport did not differ from the Ktapp value for metformin transport, the IC50 value for MPP's inhibition of metformin transport was less than its Ktapp value for transport. The simplest model to account for these observations required the influence of a distinct inhibitory site for MPP that, when occupied, decreases the translocation of substrate. These observations underscore the complexity of ligand interaction with OCT2 and argue for use of multiple substrates to obtain the needed kinetic assessment of NME interactions with OCT2.