Efavirenz has the potential to decrease ketoconazole concentrations. Data from HIV+ Thai patients showed that efavirenz decreased ketoconazole AUC by 72% and Cmax by 44%. Coadministration is not recommended as decreased ketoconazole concentrations may reduce its efficacy. Ketoconazole has a conditional risk of QT prolongation and/or TdP on the CredibleMeds.org website. Efavirenz was shown to prolong the QT interval above the regulatory threshold of concern in homozygous carriers of the CYP2B6*6/*6 allele (i.e. 516T variant in the gene encoding CYP2B6). The European product label for efavirenz contraindicates coadministration with a drug with a known risk of Torsade de Pointes whereas the American product label for efavirenz recommends that alternatives should be considered.