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 who are treated with nortriptyline may have a decreased likelihood of treatment side effects 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. However, conflicting evidence has been reported. Other genetic and clinical factors may also influence treatment related side effects.