One has to be careful with megadoses of magnesium.

1) you should have a renal panel done to make sure your kidneys are working right. Toxicity with magnesium is rare, but typically comes in renally impaired patients.

2) also some supplements are useless ...magnesium oxide is one of those. It is not appreciably absorbed at all.

I have a long detailed magnesium thread here:

http://neurotalk.psychcentral.com/thread1138.html

Please read it carefully, and especially the part about ELEMENTAL values. People and doctors get confused about this.
For example using magnesium oxide... which is about 40% magnesium: 400mg of this will yield about 8mg absorbed or thereabouts. It is the lowest bioavailability on the market.

The NIH link on my thread above, gives percentages for some common chelates.
Some labels today are including elemental values, but not all.
If you have a small tablet or capsule of chelate, don't expect much in the way of absorption...a magnesium chelate will be huge. 1000mg of magnesium malate for example delivers about 150mg of elemental magnesium and it is a huge tablet! Not all magnesium from any tablet is absorbed...they are all less than 100% and magnesium oxide is at the very bottom. Not all the calcium you consume is absorbed either, it is at 20-25% depending on the form taken. These heavy metals just do not have 100% absorption. If you find you are having looser stools (harder to notice if you use narcotics) you are losing magnesium that way.

I use Ionic Fizz often because I can adjust my dose easily. I get loose stools from magnesium commonly because of a GI birth defect I have. I really like the Ionic Fizz because it is solubilized and works very quickly, and you can adjust your dose more easily.

At the end of the thread, is a recipe for making your own magnesium cream. This looks very promising for RSD patients, where you could apply it to the affected area and improve circulation quickly that way. Soaking in Epsom salts also helps. We do that on PN commonly.