Coadministration of omeprazole (20 mg single dose) and efavirenz (600 mg single dose and 600 mg once daily) showed that when compared to values following a single dose of efavirenz, multiple doses of efavirenz decreased omeprazole AUC by ~47% (from 1843 ± 1857 nmol.h/l to 988 ± 905 nmol.h/l; (mean ± sd; n=57). A wide inter-subject variability was observed which could be partially due to polymorphisms within CYP2C19 (and potentially CYP3A5) genes. The decrease in omeprazole exposure could be relevant in some clinical situations, for example when treating helicobacter, and a dose increase of omeprazole may be required. Omeprazole 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.