Effectiveness/risks to riomet/metformin in CPP

Our daughter's endocrin put her on riomet to prevent menstruation from starting becuase her bone age is so advanced it is unclear if Lupron will work.  She has no other signs of diabetes insofar as she is not overweight, blood sugar OK, etc.
Riomet is a diabetes medication used off label for precocious puberty.
My questions are, if you use this medication for years to stave off periods, what happens to your body (beside onset of menstruation) after years of it helping you process glucose more effectively?

I asked her doctor and they don't have a real answer.  People who have gone off riomet typically do so to go onto a heavier duty diabetes medication.  This off-label use only has about a 4 year history.