The CYP2D6*5 allele has been assigned as a no function allele by CPIC. Patients carrying the *5 allele in combination with a decreased, normal or no function allele or an increased function allele with an activity value of 2 may have a similar response to ondansetron as compared to patients with alleles that result in a normal metabolizer phenotype, while patients carrying the *5 allele in combination with an increased allele with an activity value of 3 or greater may have a decreased response to ondansetron as compared to patients with alleles that result in a normal metabolizer phenotype. However, conflicting evidence has been reported. Be aware that the CPIC guideline for ondansetron and CYP2D6 has a 'no recommendation' for CYP2D6 intermediate and poor metabolizers. Other genetic and clinical factors may also influence response to ondansetron.