The CYP2D6*3 allele has been assigned as a no function allele by CPIC. Patients carrying the *3 allele in combination with a decreased, normal or no function allele may have decreased metabolism of tramadol as compared to patients with alleles that result in a normal metabolizer phenotype. Patients carrying the *3 allele in combination with an increased function allele with an activity value of 2 may have similar metabolism of tramadol as compared to patients with alleles that result in a normal metabolizer phenotype, while patients carrying the *3 allele in combination with an increased function allele with an activity value of 3 or greater may have increased metabolism of tramadol as compared to patients with alleles that result in a normal metabolizer phenotype. However, conflicting evidence has been reported. This annotation only covers the pharmacokinetic relationship between CYP2D6 and tramadol and does not include evidence about clinical outcomes. Other genetic and clinical factors may also influence tramadol metabolism.