The limitations of currently available oral antidiabetic agents either in terms of efficacy/safety coupled with the emergence of the disease into global epidemic have encouraged alternative therapy that can manage diabetes more efficiently and safely. Herbal medications are the most commonly domestic medicines accepted as alternative therapy for blood sugar control; however, their safety and efficacy need to be further evaluated by well-designed, controlled clinical studies. Because various non-standardized forms of the herbs have often been the testing material, the results have been difficult to replicate. Therefore, preparation of standardized medicinal herbs is urgently needed in future studies and therapies. Though the herbs used for the treatment of diabetes are thought to have less drawbacks, potential adverse herbdrug interactions should be kept in mind for patients also receiving conventional antidiabetic medications. The present study was designed to investigate the possible herb drug interaction between a proprietary polyherbal formulation BSL- 150 and metformin in streptozotocin-nicotinamide (STZ-NIC) induced diabetes induced in mice. Metformin was given orally at a dose of 500 mg/kg and BSL-150 was administered at a dose 250 mg/kg. The effect of single and repeated oral administration of BSL-150 and metformin on blood glucose level, serum and pancreatic insulin, liver glycogen content and glycosylated haemoglobin was evaluated. The significant (P<0.001) reduction in the blood glucose, liver glycogen content and glycosylated haemoglobin while no significant change in serum and pancreatic insulin level in combination treated group was observed as compared to the group treated with metformin alone.