I'm sure your doctor has already pointed this one out, but make sure the Magnesium supplements are not too strong. Excess Magnesium is not good.
I have a freind, who as treatment for mild mercury poisoning, had to take Magnesium. She was taking 4 tablets a day as treatment, and a further 14 to control the side effects. As she is currently writing her pathology thesis, I belive her when she states they're all necessary.