The purpose of this study was to evaluate in vitro interactions of commercially obtained pure herbal constituents with p-glycoprotein P-gp and cytochrome P-450 3A4 (CYP3A4) activities, which can further modulate the transcellular transport and metabolism kinetics of orally administered drugs. Caco-2 cells grown in the presence of 0.25 micromol/L 1alpha,25-dihydroxy vitamin D3 and multidrug-resistant 1 (MDR1) transfected MDCK cells were used as models to evaluate the effect of purified herbal constituents (quercetin, hypericin, hyperforin from St. John's wort, kaempferol from ginseng, silibinin from milk thistle, and allicin from garlic) on P-gp-mediated efflux of the human immunodeficiency virus (HIV) protease inhibitor ritonavir. In addition, the inhibitory effect of these constituents on CYP3A4-mediated metabolism was determined by using cortisol as a model compound. Silibinin and hyperforin did not significantly alter cellular uptake of H-ritonavir in Caco-2 cells. A similar result was also observed for silibinin when tested in MDR1-MDCK cells. Quercetin, hypericin, and kaempferol exhibited a remarkable inhibition of P-gp-mediated efflux of ritonavir by increasing its cellular uptake in these models. These values were also comparable with the inhibitory effect of quinidine in Caco-2 cells, a well-known inhibitor of P-gp, on ritonavir efflux from Caco-2 cells. Allicin exhibited a concentration-dependent inhibition of ritonavir efflux when tested on MDR1-MDCK cells. There was a significant decrease in the Apical to Basal/Basal to Apical (AP-BL/BL-AP) transport ratio of ritonavir in presence of hypericin, kaempferol, and quercetin. These herbal constituents inhibited the CYP3A4 activity when tested with the Vivid CYP3A4 assay kit, whereas silibinin did not alter cortisol metabolism. Hypericin showed a significant inhibition in reduced nicotinamide adenine dinucleotide phosphate (NADPH)-dependent metabolism of cortisol with 64.6% of intact drug at the end of a 1-hour study. Similarly, kaempferol and quercetin also caused substantial inhibition of cortisol metabolism with 89.7% and 90.1% of intact cortisol, respectively, compared with 45.9% in the control. Prolonged exposure of quercetin resulted in significant increase of mRNA expression of both MDR1 and CYP3A4 levels in Caco-2 cells. However, hyperforin caused upregulation of CYP3A4 and downregulation of MDR1, whereas the effect of silibinin and kaempferol remained inconclusive on these gene expressions. Hypericin, kaempferol, quercetin, and allicin inhibit the efflux and CYP3A4-mediated metabolism of xenobiotics in vitro. Hence, this study warns against the use of herbal constituents along with prescribed HIV protease inhibitors that are substrates for P-gp and/or CYP3A4.