The CYP2D6*4xN allele (*4x2) is assigned as a no function allele by CPIC. Patients carrying the *4xN allele in combination with another no function allele who are treated with atomoxetine may have an increased risk for treatment related side effects as compared to patients with alleles that result in a normal metabolizer phenotype. Other genetic and clinical factors may also influence response to atomoxetine.