CYP2C19*18 is assigned normal function based on limited evidence. Four in vitro studies found CYP2C19*18 demonstrated similar intrinsic clearance compared to wildtype with mephenytoin, omeprazole, and clopidogrel substrates (17455109, 18312490, 25001882, 21325430). Therefore, consensus among experts was normal function based on limited evidence as it was determined the phenotypes containing this allele would be clinically similar to phenotypes comprising normal function alleles.