CYP2C9*28 is assigned decreased function based on limited evidence. Multiple in vitro studies found CYP2C9*28 demonstrated decreased clearance compared to wildtype. These studies used different cell systems and substrates included warfarin, tolbutamide, diclofenac, losartan, and glimepiride (23752738, 19541829, 16788382). 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.