This link illustrates a test done on oral vs injectable in both intrinsic factor intact people and those lacking intrinsic factor (pernicious anemia). It used to be thought you had to have IF in order to use oral, but that is not the case:

http://www.ncbi.nlm.nih.gov/pmc/arti...able/T1/#TF1-4

This is the full article:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2532799/
The full paper mentions a down side to using hydroxocobalamin chronically, as potentially allergenic. That is something I have not seen before.

This link illustrates that in 1000micrograms orally used
approx 13 micrograms were absorbed, and in the patients lacking intrinsic factor, approx 12 micrograms were absorbed.
Essentially no difference.
Injection wise out of 1000 micrograms injected, 150 micrograms were measured. Far from the whole injection amount. I think doctors assume 1000 mcg are used by the body when injected, and that is not accurate.

The B12 in an oral dose is passively absorbed in the small intestine. The only way you cannot absorb it is if you have had all your small intestine removed...and that is certainly not common.

So now the doctors who treat based on this NEW evidence (2008) often allow oral to be used by patients. The only caveat I have is that you should do this on an empty stomach for best results. Presence of food reduces this process of absorption and some kinds of fiber will completely block passive absorption.