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 codeine as compared to patients carrying two increased function alleles or an increased function allele in combination with a normal function allele or a decreased function allele with an activity value of 0.5. Patients carrying the *2 allele in combination with alleles that result in a normal metabolizer phenotype may also have decreased metabolism of codeine 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 with an activity value of 0.25 but increased metabolism of codeine 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 codeine and does not include evidence about clinical outcomes. Other genetic and clinical factors may also influence codeine metabolism.