Magnesium oxide hurts my kidneys - help!

I'm taking Oxypowder , and have to take a lot of extra magnesium oxide in order to push all the liquid waste out of my body. But since doing this, my kidneys are killing me! I know I already have weak kidneys, so this concerns me. I've tried your suggestions of drinking warm lemon water, jumping up and down, etc..., but I have hard, dry feces if I don't take Oxypowder - it's the only thing that works for me - but only if I take magnesium oxide. Not sure what to do anymore.

And a question about magnesium - are all forms of magnesiums laxatives?