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Post # 10 in this thread has the links explaining absorption of oral products.

All oral magnesium has flaws. Some just more than others.
Magnesium oxide does not ionize in GI tract so passes mostly thru to become a laxative. Phillips caplets contain this form and are advertised as a "gentle" overnight laxative. The magnesium stearate in vitamins is also inert and passes thru. Many people see stearate listed on labels and think they are getting magnesium, but they are not.

Minerals can be blocked during absorption if certain drugs are taken. Acid blocking drugs for GERD, will reduce some magnesium absorption, for example. If you listen carefully to the commercials on TV, they even state that now. But for many years this was not understood well. Also if a person has Celiac or gluten intolerance magnesium will be poorly absorbed no matter what form it is in.

Diuretics deplete magnesium and many drugs affect its utilization in some ways...hormone treatments, long term antibiotics (as in Lyme treatments), and steroid treatments will affect magnesium in the body. (this information is from Drug Induced Nutrient Depletion Handbook, by Pelton and LaValle, 2nd ed.)

Personally I've been using magnesium in a new lotion called Morton's Epsom Lotion. This has been the best for me. It even reduces pain levels, and my hypertension.
http://www.mortonsalt.com/for-your-h...-epsom-lotion/
This is still new but available inexpensively at WalMart and Amazon. I expect to see it at more sellers soon. It is just fabulous and I no longer take oral supplements myself. And I've reduced my hypertension medication from 3 to one small dose.
It is really good for muscle strains, tendon problems, or nerve pain. When people have circulation issues, where the small blood vessels are constricted the topical works much better than oral. The mag in the blood really can't get to places, where circulation is impaired. Inflammation, for example, that is long term does this or oral is hampered whereas topical bypasses that issue.

This is a long thread, and encompasses many years--since 2006. I put up scientific papers when I find them on PubMed...and so one has to read along to find those. Medical topics tend to have conflicting studies at times like most other medical information, so NOTHING is caste in stone with supplements, drugs, devices, or treatments. It has been relatively recent that studies on bioavailability have been done at all.