I've been taking metformin for about 20 years now. I started with 500mg 2x per day and eventually the dose was increased to 850 mg 2x/day.
I know it causes gastric problems with some people but I never had any trouble with it at all. For people who do, there's a time-release version that's supposed to cause less trouble. And if it still is a problem, there are other drugs. But metformin is always what they start you on because it works great, it's been around a long time, and it's available as a cheap generic (I pay $12 for 3 months worth! I dropped my insurance prescription coverage because their $20 co-pay is actually more than the cost of the drug!)
Now I'm diabetic, as you might have guessed already. Type 2. A few years ago I wondered if metformin even did anything for me, considering that I do a pretty good job of controlling my blood sugar through diet and exercise (and that I've lost a lot of weight, as my Dr. recommended). So I went off it for a week or so, just to see what effect it would have on my blood sugar readings. It made a BIG difference! My post-prandial readings shot up about 40 points! So I know it works.