Been on Metformin 16 days. Noticing light headedness. Was diagnosed with prediabetes in March. A1C was 6.5. Doctor prescribed Metformin however, I wanted to try and work on myself with diet. After taking a class to educate myself, that is when I decided to start Metformin. So far, that is the only side affect that I have notice. I do have loose bowels but nothing serious.