In continuation of my update on Metformin... 
Kinesiology researcher Barry Braun of the University of Massachusetts  Amherst and colleagues recently reported unexpected results of a study  suggesting that exercise and one of the most commonly prescribed drugs  for diabetes, metformin, each improves insulin resistance when used  alone, but when used together, metformin blunted the full effect of a  12-week exercise program in pre-diabetic men and women.
Ref : http://www.umass.edu/newsoffice/newsreleases/articles/142505.php