The CYP2D6*1 allele is assigned as a normal function allele by CPIC. Patients carrying the CYP2D6*1 allele in combination with alleles that result in a normal metabolizer phenotype may have increased metabolism of nortriptyline as compared to patients with a no function allele in combination with a decreased or normal function allele or two no or decreased function alleles. Patients carrying the CYP2D6*1 allele in combination with alleles that result in a normal metabolizer phenotype may have decreased metabolism of nortriptyline as compared to patients with an increased function allele in combination with an increased or normal function allele or a decreased function allele with an activity value of 0.5. Patients carrying the CYP2D6*1 allele in combination with alleles that result in a normal metabolizer phenotype may also have decreased metabolism of nortriptyline 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. However, conflicting evidence has been reported. This annotation only covers the pharmacokinetic relationship between CYP2D6 and nortriptyline and does not include evidence about clinical outcomes. Other genetic and clinical factors may also influence metabolism of nortriptyline.