There is still an argument about ginseng-prescription drug interactions. To evaluate the influence on cytochrome P450 (P450) activities of ginseng in the present study, the influence on P450 activities of naturally occurring ginsenosides and their degradation products in human gut lumen was assayed by using human liver microsomes and cDNA-expressed CYP3A4. The results showed that the naturally occurring ginsenosides exhibited no inhibition or weak inhibition against human CYP3A4, CYP2D6, CYP2C9, CYP2A6, or CYP1A2 activities; however, their main intestinal metabolites demonstrated a wide range of inhibition of the P450-mediated metabolism. There was no mechanism-based inhibition found on these P450 isoforms. It is noteworthy that Compound K, protopanaxadiol (Ppd), and protopanaxatriol (Ppt) all exhibited moderate inhibition against CYP2C9 activity, and Ppd and Ppt also exhibited potent competitive inhibition against CYP3A4 activity. We suggest that after oral administration, naturally occurring ginsenosides might influence hepatic P450 activity in vivo via their intestinal metabolites.