Its primary effect is to inhibit the liver's production of glucose and, possibly, to stimulate the process of transporting glucose into muscle, a process which requires insulin. Thus it only works when there is insulin around, for example in type 2 diabetes, but not type 1 diabetes, which is characterized by insulin deficiency. Exactly how metformin works is not well understood. Metformin can be used as a first line of therapy. Metformin is also as capable as the sulfonylureas in reducing HbA1c.
Side effects can be a problem with metformin.
The largest concern with metformin is the potential to produce a build up of lactic acid. However, this is a very rare side effect of the drug, particularly if care is taken not to prescribe metformin when it is contraindicated. Contraindications for this drug include evidence of kidney disease, significant liver disease, chronic alcoholism or congestive heart failure. Hypoglycemia and, as mentioned above, weight gain, are not on the list of metformin's side effects. TZDs are effective in reducing HbA1c.