Both the OTC herbal supplement Berberine and the prescription medication Metformin are effective at lowering blood glucose levels, as measured by HA1C. New studies are now suggesting that these two substances exert their positive effects on blood sugar most likely by the same mechanism: They both have a favorable or balancing effect on gut bacteria. This highlights the importance of the gut environment and it's relationship to metabolic diseases such as diabetes.
Zhang Q, Li Y, Chen L.[Effect of berberine in treating type 2 diabetes mellitus and complications and its relevant mechanisms].Zhongguo Zhong Yao Za Zhi. 2015 May;40(9):1660-5.