Coadministration of famotidine (40 mg single dose) and efavirenz (400 mg single dose) had no effect on efavirenz Cmax or AUC. Famotidine 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.

Coadministration has not been studied but based on metabolism and clearance a clinically significant interaction is unlikely. Metabolism of phytomenadione is thought to involve CYP4F2 and glucuronidation. Renal elimination of phytomenadione is minimal.