Fig. 4. Effects of CYP2D6 genotype on CYP2D6 substrate metabolism. Extensive metabolizers have one or two normal copies of the CYP2D6 gene and are considered wild-type. A normal β-blocker dose will likely result in a therapeutic drug concentration. Ultra-rapid metabolizers possess more than 2 functional copies (up to 13). A CYP2D6 substrate (e.g. , metoprolol) will be rapidly metabolized and the drug effect minimal. Intermediate metabolizers have one or two hypofunctional CYP2D6 alleles, which result in a reduced CYP2D6 function. Substrate metabolism is reduced and higher drug concentrations result with the possibility of an exaggerated response. Poor metabolizers have two nonfunctional copies of CYP2D6, which result in a nonfunctional enzyme. This may lead to toxic drug concentrations.