The CYP2B6*6 allele is assigned as a decreased function allele by CPIC. Patients carrying the CYP2B6*6 allele in combination with a normal, decreased, no, or increased function allele may have increased risk of adverse events (eg. liver toxicity or CNS side effects) when treated with efavirenz as compared to patients with two normal function alleles. However, conflicting evidence has been reported. Other genetic and clinical factors may also influence toxicity of efavirenz.