CYP2D6*10 is assigned decreased function based on strong evidence. CYP2D6*10 has consistently been described in subjects demonstrating decreased metabolism of various CYP2D6 substrates, with metabolic ratios greater than those observed in wildtype subjects but less than poor metabolizers (8764380, 8287064, 19364831). In vitro studies using different cell systems similarly found CYP2D6*10 demonstrates decreased intrinsic clearance compared to wildtype with various substrates (27538917, 27354764, 27251229, 27203132, 26968424, 26937172, 26652007, 26542054, 26310775, 25469868, 30366777, 24647041, 11266079, 18784265). As part of the standardization of CYP2D6 genotype to phenotype translation among CPIC and the Dutch Pharmacogenetics Working Group, CYP2D6*10 was reevaluated as the evidence suggests the activity of the CYP2D6*10 is considerably lower compared to other decreased function alleles and the activity value of 0.5 (previously assigned) may overestimate metabolic capacity of patients with this allele (31647186). Therefore, consensus among experts was decreased function with an activity value of 0.25 based on strong evidence.