The CYP2C9*1 allele has been assigned as a normal function allele by CPIC. Patients carrying the *1 allele in combination with another normal function allele may have increased metabolism/clearance of phenytoin as compared to patients with a normal function allele in combination with a no or decreased function allele, two no or decreased function alleles, or one decreased function allele in combination with a no function allele. This annotation only covers the pharmacokinetic relationship between CYP2C9 and phenytoin and does not include evidence about clinical outcomes. However, conflicting evidence has been reported. Other genetic and clinical factors may also influence phenytoin metabolism.