The CYP2D6*10 allele is assigned as a decreased function allele with an activity value of 0.25 by CPIC. Patients carrying the CYP2D6*10 allele in combination with a no or decreased function allele who are treated with nortriptyline may have an increased likelihood of treatment side effects as compared to patients with alleles that result in a normal metabolizer phenotype. However, conflicting evidence has been reported. Other genetic and clinical factors may also influence treatment related side effects.