Patients with one copy of the *17 allele in combination with one copy of the *1A allele may have decreased metabolism of metronidazole as compared to patients with two copies of the *1A allele. However, one study has failed to find this association. 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.