I use magnesium threonate and have no ill effects. It's a newer type of mag. that is supposed to be be highly absorbable. It's also touted as the only form of mag. that crosses the blood-brain barrier. You can read more about this at the Mercola and other websites.
One note on the magnesium test mentioned in the prior entry: Magnesium is best measured using the mag. RBC test, which measures the amount of mag. within red blood cells, which is where you want it to be. On the other hand, the magnesium serum test measures mag. floating around in your blood, which isn't valuable info.
Rose