The CYP2C9*11 allele has been assigned as a decreased function allele by CPIC. Patients carrying the *11 allele in combination with a normal, decreased, or no function allele may have decreased metabolism/clearance of phenytoin as compared to patients with two normal function alleles. However, conflicting evidence has been reported. This annotation only covers the pharmacokinetic relationship between CYP2C9 and phenytoin and does not include evidence about clinical outcomes. Other genetic and clinical factors may also influence phenytoin metabolism.