I've used GNC and Nature Made brands of magnesium oxide. Some people have found magnesium citrate to work better for them. I would start with 250 to 400mg. Anything over 400mg I would check with your doc. It took a while for it to start working for me because it's not a quick fix like a laxative. It's a daily regime and if I miss one day of taking it, my stools are back to being hard and difficult to pass.