The CYP2D6*10 allele is assigned as a decreased function allele with an activity value of 0.25 by CPIC. Patients carrying the *10 allele in combination with a decreased or no function allele may have a decreased analgesic response when treated with tramadol as compared to patients with alleles that result in a normal metabolizer phenotype. Patients carrying the *10 allele in combination with a normal function allele or an increased function allele with an activity value of 2 may have a similar analgesic response when treated with tramadol as compared to patients with alleles that result in a normal metabolizer phenotype, while patients carrying the *10 allele in combination with an increased function allele with an activity value of 3 or greater may have an increased analgesic response when treated with tramadol 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 tramadol.