Even though magnesium oxide is the most available form of magnesium, it is not very effective because only about 5% gets absorbed . Magnesium oxide supplements are popular because they were one of the first forms to hit the market and the pills are smaller. They appear to contain more magnesium, but if you cannot absorb it, it will not help most people.
If you want to use a magnesium supplement, then you are better off using a form like magnesium citrate or magnesium chloride.
The majority of magnesium oxide produced today is obtained from the naturally occurring minerals such as magnesite, MgCO3 which is process by calcinations. Another important source of magnesium oxide is seawater, underground deposits of deep salt beds and brine from which magnesium hydroxide Mg(OH)2 is processed.
There are many commercial uses for Magnesium Oxide including making refractories, cement, heating elements, thermocouples, brake linings and plasma screen displays.
Magnesium Oxide is often used as a Constipation Relief because it is not easily absorbed by your body.
What doesn't get absorbed by your body for nutritional uses, gets deposited into your colon.
Magnesium Oxide has also been used for lots of medical research project because it has been along so long. The interesting thing is that even though it has a low absorption rate, it still shows good efficacy. Just think how much better results you would get from products like Magnesium Citrate or Magnesium Chloride.