The intestinal bacteria, Eubacterium sp. and Bifidobacterium sp., participate in the metabolism of active kampo-ingredients, glycyrrhizin (GL), sennoside (SEN) and baicalin (BL). Since antibiotics and bacterial preparations, Bifidobacterium longum (LAC-B), Clostridium butyricum (MIYA-BM), and Streptococcus faecalis (BIOFERMIN), affect the bacterial population in intestinal bacterial flora, metabolism of the active kampo-ingredients in the bacterial flora may be altered by their combined administration. We investigated 1199 prescriptions including kampo-medicines for 308 patients. Combination use of kampo-medicines with antibiotics and bacterial preparations occurred with 7% and 10% of the kampo-prescription, respectively. Most antibiotics have activity against intestinal bacteria, except that cephems and macrolides are not active against to E. coli. This means that antibiotics may lower the metabolism of GL, SEN and BL when administered in combination. On the other hand, it is also highly possible that bacterial preparations increase the number of Eubacterium sp. and Bifidobacterium sp., resulting in enhanced metabolism of GL and SEN when they are used concomitantly with kampo-medicines. The present results suggested that the drug interactions of kampo-medicines with antibiotics and bacterial preparations should be confirmed in clinical studies.