The CYP2D6*4 allele has been assigned as a no function allele by CPIC. Patients carrying the *4 allele in combination with a decreased, normal or no function allele may may be more likely to develop side effects when treated with venlafaxine 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 likelihood of experiencing side effects when treated with venlafaxine.