I, too, am on 500 of metformin. I believe my internist put me on it to shut me up because I was complaining that I couldn’t manage my diabetes on exercise and diet alone anymore (not unless I wanted to eat 0 carbs! and run 5 miles everyday). When I finally went to an endo, he put me on Byetta, which has been managing my sugars WAY more effectively. He still kept me on the metformin, but I see him again on Thursday, so we’ll most likely talk about that. I, too, had horrible diarrhea from the metformin when I ate anything resembling a refined carb.
I have my gall bladder out, so that was an issue as well - the first time I ate a Burger King fry after my cholecystecomy, I almost lost it!
If you want to prove to your doctor that it is ineffective, this is what I did. I tested six-eight times a day. You can really see where your diet is controlling you that way. When you keep a food log (I know, I know) and test, the proof is right there and cannot be denied.
When I showed my endo that my fastings even on Met were over 125 plus sometimes 3 hours pp’s of 180 after eating a simple meal or one piece of candy over Christmas time, he knew to adjust me to something else to help out my pancreas.
The only reason I’m still on Metformin is to help out my liver as well.