Ginseng, a commonly used natural product, has been frequently reported to induce herb–drug interaction with many clinical drugs. The intestinal bacterial metabolites of ginsenosides have been widely regarded as the substance basis for ginseng–drug interactions. To date, little is known about the inhibitory effect of intestinal bacterial metabolites of ginsenosides towards UDP-glucuronosyltransferases (UGTs). In vitro investigation of the inhibition of 20(S)-protopanaxatriol (ppt) towards UGT1A1 and UGT2B7 was carried out. The results showed that ppt exhibited strong noncompetitive inhibition towards UGT1A1 and competitive inhibition towards UGT2B7. The inhibition kinetic parameters (Ki) were calculated to be 8.8 and 2.2 μM for UGT1A1 and UGT2B7, respectively. Using the maximum plasma concentration of ppt, the alteration of area under the concentration–time curve was calculated to be 20% and 70% respectively for UGT1A1-mediated and UGT2B7-mediated metabolism. However, given that the varied contribution of these two UGT isoforms towards drug metabolism and the influence of herb complexity and individual difference, the explanation of these results should be paid more caution. Copyright © 2012 John Wiley & Sons, Ltd.