2018). The inhibition of CYP2D6 could increase the oral bioavailability of various drugs and the risk of cancer (Mittal et al. 2015; Athukuri and Neerati 2016; Peterson et al. 2019). We have shown here that GAA inhibited CYP2D6-catalyzed dextromethorphan O-demethylation as a competitive inhibitor, though it is not a specific substrate of CYP2D6. This evidence concerns the gene CYP2D6 and cancer.