B12 requires intrinsic factor for absorption from food. The amounts in food are really small...like 2 micrograms a serving or less. The ACID from the stomach is the first step..breaking the protein apart so the rest can happen.

Acid blocking drugs stop this...so any B12 in food cannot be broken down for further absorption.

So taking a very large relative amount of it in an oral tablet,
bypasses the need for the stomach to break down protein.
(in fact people using acid blockers also lose absorption of
calcium, zinc, magnesium, iron and folic acid as well).
Out of a 1000 microgram dose (1milligram) about 10% is
PASSIVELY absorbed and instrinsic factor is not needed either.
So oral has to be daily in order to work. Because so little is absorbed,
it has to be given on an empty stomach, because food will act like a
sponge in the GI tract and prevent it from getting into the blood stream.

Injections can be given. If a person is really low, they stack them at first like daily for a week, or whatever, then once a week or once a month. It is important to find out the NUMBER from the test. Anything below 500 should be treated.
The U.S. is way behind in this, and often very low numbers are called "normal"...200, 250, etc. I worked in long term care and just about every patient got B12.

Read these:
http://www.ncbi.nlm.nih.gov/pubmed/10448529

http://www.aging-parents-and-elder-c...eficiency.html

If the blood test is low, it is a good idea to do a MMA blood test. Some nursing homes are beginning to do this upon admission.

There are no risks with B12, and it has no toxicity that has ever been reported.

When elderly have so many drugs...you also need to keep getting kidney tests.
Any change in function, may cause the drugs to build up in the system and
then they cause all sorts of problems.
I hope you are testing her blood sugar regularly too.