They can cause digestive problems for some people and not for others. If you have problems tell your doctor and see if you can try extended release metformin. Some people cannot tolerate it in any form.
There is 2 types of Metfornin, the docs give you the cheaper type which can cause acid reflux for sone, it did for me, go back and they will give you the more expensive type which is way better.
Most people find that if they start with a small dose of Metformin and slowly increase the stomach problems don't last for long. Its the people with the problems that post most often to forums.....
The slow release Metformins often work for poeple you can't get on with the normal Metformin, but they don't give as good results, therefore give your body time to get used to the normal type before giving up on them.
I have had no problem with it, my GP put me on Metformin XR 2 x 500 mg tablets daily with food. At the time XR was not available as it was in short supply here in Australia, so I started out on the quick release Metformin.
No stomach problems, or any of the side effects others complain about.