The CYP2D6*2 allele is assigned as a normal function allele by CPIC. Patients carrying the *2 allele in combination with alleles that result in a normal metabolizer phenotype may have decreased metabolism of metoprolol as compared to patients carrying two increased function alleles or an increased function allele in combination with a normal function allele. Patients carrying the *2 allele in combination with alleles that result in a normal metabolizer phenotype may also have decreased metabolism of metoprolol as compared to patients carrying an increased function allele with an activity value of 3 or greater in combination with a no function allele or a decreased function allele but increased metabolism of metoprolol as compared to patients with a no function allele in combination with a decreased or normal function allele or two decreased or no function alleles. This annotation only covers the pharmacokinetic relationship between CYP2D6 and metoprolol and does not include evidence about clinical outcomes. Other genetic and clinical factors may also influence metoprolol metabolism.