Do you take anything in addition to the magnesium for your "C"? I would try going back to 400 milligrams and adding a fiber supplement with it, or some people have had good luck with olive oil. I don't think taking 800 milligrams is a good idea, because a lot of what we eat and drink also contains some magnesium. I know a lot of people have complained about increased gas after taking magnesium, but it has never caused me any more gas than I usually have, although I never go over the 400 milligrams.