CYP2C19*8 is assigned no function based on limited evidence in a heterozygous subject and in vitro studies. CYP2C19*8 was first described in a subject demonstrating decreased metabolism of mephenytoin based on urine metabolite concentrations. This correlated with an in vitro study which found CYP2C19*8 had decreased enzyme activity compared to wildtype with mephenytoin and tolbutamide substrates (10411572). Two additional in vitro studies using different cell systems found CYP2C19*8 had nondetectable enzyme activity with mephenytoin, omeprazole, and clopidogrel substrates (21325430, 25001882). Another study found CYP2C19*8 exhibited significantly reduced catalytic activities compared with CYP2C19*1 across multiple substrates and distinct function from other decreased function alleles (21692664). Therefore, consensus among experts was no function based on limited evidence as it was determined the phenotypes containing this allele would be clinically distinct from phenotypes comprising normal function and decreased function alleles.