Patients carrying one copy of the CYP2A6*23 allele in combination with one copy of the *1A allele may have decreased metabolism of nicotine as compared to patients with two copies of the *1A allele or patients with one copy of the *1A allele in combination with the *9 or *12 alleles. This annotation only covers the pharmacokinetic relationship between CYP2A6 and nicotine and does not include evidence about clinical outcomes. Other genetic and clinical factors may also affect nicotine metabolism.