The CYP2D6*6 allele is assigned as a no function allele by CPIC. Patients carrying the CYP2D6*6 allele in combination with a no, decreased or normal 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.