CYP2C9*11 is assigned decreased function based on definitive evidence in heterozygous patients and in vitro studies. Four subjects carrying the CYP2C9*11 allele (n=3 CYP2C9*1/*11, n=1 CYP2C9*8/*11) demonstrated decreased phenytoin metabolism compared to wildtype subjects based on urinary excretion of a phenytoin metabolite after a single 300mg dose (16220110). Three CYP2C9*1/*11 patients on a stable dose of warfarin required a 33% lower warfarin dose compared to CYP2C9*1/*1 patients which correlated with the in vitro investigation where CYP2C9*11 had low levels of enzyme expression, decreased half-life, and 70% catalytic efficiency compared to wildtype; these results are suggestive of improper protein folding leading to decreased stability and reduced enzyme activity (15970795). An examination of the crystal structure showed the substitution which defines the CYP2C9*11 allele disrupts the sole extrahelical hydrogen bonding interaction, destabilizing the structure (15284535). In vitro studies showed CYP2C9*11 demonstrated decreased metabolism of tolbutamide, warfarin, glimepiride, carvedilol, fluoxetine, flurbiprofen, mestranol, diclofenac, and losartan compared to wildtype (15284535, 23752738, 24118918, 25476996, 25884291, 27199745, 25144335, 25924705, 25951663, 23844998). Although other in vitro studies showed CYP2C9*11 had similar activity compared to wildtype, the experts did not find these results convincing as in these studies CYP2C9*2 and/or CYP2C9*3 which definitively have decreased function and no function, respectively, also demonstrated similar activity compared to wildtype in these studies (25142737, 25832633, 27163851). The experts did not find the result of an in vitro study which showed CYP2C9*11 had similar activity compared to wildtype with a tolbutamide substrate (24077631) convincing because this result was not replicated as two separate in vitro studies which used two different systems and showed CYP2C9*11 demonstrated decreased clearance compared to wildtype, as described previously (15284535, 23752738). The experts also did not deem the results of a study which found no difference in losartan metabolism among four subjects carrying the CYP2C9*11 allele nor a study which found no difference in acenocoumarol metabolism among one subject with CYP2C9*1/*11 genotype as convincing conflicting evidence due to the small sample size (15289788, 21811894). Therefore, consensus among experts was decreased function with an activity value of 0.5 based on definitive evidence.