Patients with two copies of the *1A allele may have increased metabolism of metronidazole as compared to patients with one copy of the *1A allele in combination with one copy of the *2, *9 or *17 alleles. However, one study found failed to find this association for the *17 allele. Other genetic and clinical factors may also affect metronidazole metabolism. This annotation only covers the pharmacokinetic relationship between CYP2A6 and metronidazole and does not include evidence on clinical outcomes.