Carbon nanostructures have recently gained significant interest from scientists due to their unique physicochemical properties and low toxicity. They can accumulate in the liver, which is the main expression site of cytochrome P450 (CYP450) enzymes. These enzymes play an important role in the metabolism of exogenous compounds, such as drugs and xenobiotics. Altered activity or expression of CYP450 enzymes may lead to adverse drug eects and toxicity. The objective of this study was to evaluate the influence of three carbon nanostructures on the activity and expression at the mRNA and protein levels of CYP2C9 isoenzyme from the CYP2C subfamily: Diamond nanoparticles, graphite nanoparticles, and graphene oxide platelets. The experiments were conducted using two in vitro models. A microsome model was used to assess the influence of the three-carbon nanostructures on the activity of the CYP2C9 isoenzyme. The CYP2C9 gene expression at the mRNA and protein levels was determined using a hepatoma-derived cell line HepG2. The experiments have shown that all examined nanostructures inhibit the enzymatic activity of the studied isoenzymes. Moreover, a decrease in the expression at the mRNA and protein levels was also observed. This indicates that despite low toxicity, the nanostructures can alter the enzymatic function of CYP450 enzymes, and the molecular pathways involved in their expression.