Patients with two copies of the CYP2A6*4A allele or one copy of the *4A allele in combination with one copy of the *1A, *7, *9A or *17 alleles may have decreased metabolism of nicotine as compared to patients with two copies of the *1A, *14 or *38 alleles, while patients with one copy of the *4A allele in combination with one copy of the *9A allele may also have decreased metabolism of nicotine as compared to patients with one copy of the *4A allele in combination with one copy of the *1A allele. Patients with one copy of the *4A allele in combination with one copy of the *1A allele may also have decreased metabolism of nicotine as compared to patients with one copy of the *1A allele in combination with the *9, *12, *14 or *38 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.