Metformin, sold under the brand names of Bolamyn, Diagemet, Glucient, Glucophage, Metabet, Obimet, Gluformin, Dianben, Diabex, Diaformin, Metsol, Siofor, Metfogamma (and many others) is a prescription-only medication used in the treatment of Type 2 Diabetes. It has been used worldwide since 1957, and the prescription rates are now around 78 million in the US alone. It can reduce a diabetic person's (excess) blood glucose by 30%.
It was originally derived from the French lilac or Goat's rue (Galega officinalis) which have been used in herbal remedies for several centuries.
Its mechanism of action is unknown. There are multiple proposals as to how it might operate - including as an appetite suppressant. See Wikipedia