CYP2C9*12 is assigned decreased function based on limited evidence in heterozygous patients and in vitro studies. Four patients on a stable dose of warfarin with CYP2C9*1/*12 genotype required a lower weekly warfarin dose compared to CYP2C9*1/*1 patients (23688605). In vitro studies similarly found CYP2C9*12 demonstrated decreased enzyme activity with warfarin and tolbutamide substrates compared to wildtype (23752738, 15284535, 28758225). Therefore, consensus among experts was decreased function with an activity value of 0.5 based on limited evidence as it was determined the phenotypes containing this allele would be clinically distinct from phenotypes comprising normal function alleles.