The investigators will test the hypothesis that CYP2B6 genetic variants that show effects in vitro also influence the in vivo activity of CYP2B6 and its responsiveness to metabolic inhibition, using the metabolism of efavirenz (100 mg dose) as a marker of activity. The metabolism and pharmacokinetics of efavirenz will be determined in a total of 60 healthy volunteers with CYP2B6*1/*1, CYP2B6*1*6 and CYP2B6*6*6 at baseline (control) and after pretreatment with voriconazole.