Based on data with paroxetine, no pharmacokinetic interaction is expected with fluoxetine and efavirenz. Population pharmacokinetics of efavirenz in patients receiving selective serotonin reuptake inhibitors (fluoxetine, paroxetine and sertraline) showed that they did not appear to significantly affect efavirenz plasma levels. Fluoxetine 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. As the potential risk of a QT interval prolongation relates specifically to homozygous carriers of CYP2B6*6/*6 and given the accumulated years of safety data with efavirenz and such drugs, the contraindication is not reflected in the colour coding of this interaction summary.