Efficacy

Metformin has an efficacy similar to sulfonylurea agents.  Unlike the sulfonylureas, when metformin is used as monotherapy, there is potential weight loss and no risk for hypoglycemia. 

In the study shown here, patients on glyburide were either continued on that drug, changed to metformin, or had metformin added.  As you can clearly see, changing medications did not get you anywhere.  But, when the medications of these two different classes were added, there was a dramatic improvement in glucose control. 

This finding can be seen with any two classes of oral hypoglycemic medications.