The CYP2D6*1xN alleles (*1x2 and *1x鈮?) have been assigned as increased function alleles by CPIC. Patients carrying the CYP2D6*1xN allele in combination with alleles that result in a normal metabolizer phenotype who are treated with amitriptyline may have decreased likelihood of side effects as compared to patients with a combination of alleles that result in intermediate or poor metabolizer phenotype. Other genetic and clinical factors may also influence response to amitriptyline.