The CYP2D6*5 allele is assigned as a no function allele by CPIC. Patients carrying the *5 allele in combination with a decreased, normal or no function allele may have a decreased analgesic response when treated with codeine as compared to patients with alleles that result in a normal metabolizer phenotype. Patients carrying the *5 allele in combination with an increased function allele may have a similar analgesic response when treated with codeine as compared to patients with alleles that result in a normal metabolizer phenotype. Other genetic and clinical factors may also influence analgesic response when treated with codeine.