Sennoside A (SA), the main purgative constituent of Daiokanzoto (da-huang-gan-cao-tang; DKT), is generally regarded as a prodrug that is transformed into an active metabolite by β-glucosidase derived from Bifidobacterium spp. It has been suggested that antibiotics would promote dysbiosis, and thereby inhibit the purgative activity of DKT. In this study, ampicillin was administered to mice for 8 d, and the changes in the SA metabolism of SA alone and of DKT were investigated. The results showed that the SA metabolism of SA singly continued to be inhibited by ampicillin, but that of DKT was activated from day 3 under the same conditions. In order to investigate the mechanism of SA metabolism activated by DKT in the mice administered ampicillin, changes in the SA metabolism were observed in the presence of rhein 8-O-β-D-glucopyranoside (RG) in rhubarb and liquiritin in glycyrrhiza, both of which accelerated the SA metabolism. In fact, RG achieved an activation of SA metabolism similar to that by DKT. The purgative action of DKT, which was continued treatment of the ampicillin, was significantly greater than that by SA alone, and it was shown that RG was involved in this effect. We also analyzed changes in the intestinal microbiota before and after administration of ampicillin. No Bifidobacteria were detected throughout the treatment, but the population of Bacteroides was significantly increased after 3 d under the same conditions. Taken together, these results strongly suggested that the RG in DKT changed the function of Bacteroides and thereby allowed DKT to metabolize SA.