In order to quantitatively predict drug interactions associated with efavirenz-based anti-HIV therapy, we evaluated reversible and time-dependent inhibitions of efavirenz on eight cytochrome P450 (CYP) enzymes in vitro. The present study showed that efavirenz was a potent competitive inhibitor of CYP2B6 (average K(i) = 1.68 µM in HLMs and K(i) = 1.38 µM in expressed CYP2B6) and CYP2C8 (K(i) = 4.78 µM in pooled HLMs and K(i) = 4.80 µM in HLMs with CYP2C8*3/*3 genotype). Efavirenz was a moderate inhibitor of CYP2C9 (K(i) = 19.46 µM) and CYP2C19 (K(i) = 21.31 µM); and a weak inhibitor of CYP3A (K(i) = 40.33 µM). No appreciable inhibition was observed on CYP1A2, CYP2A6 or CYP2D6. No time-dependent inhibition of the CYPs by efavirenz was observed in this study. Quantitative predictions showed that single dose of efavirenz may substantially slow the elimination of drugs predominantly cleared by CYP2B6, CYP2C19 or by both enzymes and may also lower the area under the plasma concentration time curve (AUC) of active metabolites of some pro-drugs (e.g., clopidogrel and proguanil) by up to 30%. Depending on substrates, chronic administration of efavirenz may increase the AUC of CYP2C8 and CYP2C9 substrates about 3.5-4.4-fold and 1.7-2.0-fold at steady state.