Coadministration of clarithromycin (500 mg every 12 hours) and efavirenz (400 mg once daily) decreased clarithromycin Cmax (26%), AUC (39%) and Cmin (53%) and increased 14-OH clarithromycin Cmax (49%), AUC (34%) and Cmin (26%). Efavirenz Cmax increased by 11%, but AUC and Cmin were unaltered. The clinical significance of the decreases in clarithromycin is unknown. In uninfected individuals, 46% developed rash while receiving efavirenz and clarithromycin. No dose adjustment of efavirenz is recommended when given with clarithromycin. Alternatives to clarithromycin, such as azithromycin, should be considered. Clarithromycin has a known 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.