Normally if you are B12 deficient (and aren't vegan), you have pernicious anemia which is a lack of intrinsic factor. Intrinsic factor is a substance produced in the digestive tract that is required for the absorption and uptake of B12. If, in fact, you suffer from pernicious anemia, you will require B12 injections. Sometimes you can get by with megadosing B12 supplements as some with pernicious anemia still make intrinsic factor but just not in sufficient amounts. My mom has suffered with pernicious anemia most of her life and requires periodic B12 injections.
There is no replacement for B12 other than doing what it takes to get levels up to normal. In your case, it would be worthwhile to shoot for therapeutic levels for some period of time as the body plays catch up.
I should add that you should be supplementing with the active form of B12 (methyl B12 or methylcobalamin) rather than plain cyanocobalamin as you may also have a genetic polymorphism that inhibits the pathway to methylate the inactive form (cyanocobalamin) to the needed active form (methylcobalamin). Buy and try some methyl B12 (methycobalamin) which is dosed sublingually and bypasses the intrinsic factor required in oral dosing.
Edited by Hebbeh, 23 December 2013 - 09:17 PM.