The CYP2D6*1xN alleles (*1x2 and *1x鈮?) are assigned as increased function alleles by CPIC. Patients carrying the *1xN allele in combination with a normal or increased function allele or a decreased function allele with an activity value of 0.5 may have increased metabolism of codeine as compared to patients with alleles that result in a normal metabolizer phenotype. Patients carrying a *1xN allele with an activity value of 3 in combination with a decreased function allele with an activity value of 0.25 or a no function allele may also have increased metabolism of codeine as compared to patients with alleles that result in a normal metabolizer phenotype. Patients carrying a *1xN allele with an activity value of 2 in combination with a decreased function allele with an activity value of 0.25 or a no function allele may have similar metabolism of codeine as compared to patients with alleles that result in a normal metabolizer phenotype. 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.