There's a great deal of difference between various forms of magnesium. Different oral forms have different bioavailability. For example, magnesium oxide is the most poorly absorbed form of magnesium. It's often used in oral forms, and this is why the amounts of magnesium in these have to be so very high....so that enough gets absorbed.
Magnesium citrate is one of the most easily absorbed forms of magnesium, but as in all oral preparations, only about 3-=5- percent of the magnesium is absorbed at all thru the intestines. Depending on your purpose for taking magnesium, this can be a problem. If you are taking oral forms of magnesium to increase body levels of this mineral, which is being found to be deficient in over 68% of the US population......it can take six months to a year to bring levels up to where they should be, taking it everyday. One of the side effects of taking magnesium orally, is diarrhea....so doses have to be adjusted for this. There are also many intestinal problems that can interfere with the absorption of magnesium. We all need to ensure we have enough magnesium in our bodies, and it has been depleted from refined foods, our soils etc. and is not as easy to get enough in the diet alone, as it sounds. Depending on the form of magnesium used, long term use may also be a problem.
The IMVA (International Medical Veritas Association) has been investigating different forms of magnesium and has recommended that magnesium chloride is the most versatile, best absorbed, and safest in toxicity profiles. But it is recommended in the form of "transdermal" application instead of oral intake. This is a slightly oily lotion that can be applied to the skin or used in baths, foot baths....or many other ways. It is not used to treat constipation as many oral magnesium products are. It is used to increase body levels of magnesium, which is beneficial in preventing diabetes, cardiac problems, relieving stress, promoting sleep, reducing hypertension, reducing cholesterol.....and much more. It is an essential mineral that needs to be maintained at proper levels in our bodies. Studies of use of magnesium chloride oil in a foot bath with body spraying by Dr. Norm Shealy has shown that this method increases levels of magnesium faster than any oral supplements can. See: http://www.magnesiumforlife.com/relevantresearch.shtml
and visit the Magnesium For Life website to learn more.