Start with 5mg (5000mcg) daily on an empty stomach. Food reduces absorption, and oral needs high doses to get even a small amount of B12 into the bloodstream.
After 3 months, you should get retested, and see where you are.
You may see progress during that time or not, depending on how long you have been very low. After reaching 1000 or more blood levels, you can reduce to 1000mcg a day, or use your 5000 tablets up 2 or 3 times a week.
It took a long time to get where you are now. It will take time to improve because of that. You probably will need the B12 forever.
If your diet caused this, you will need to supplement daily. If you have the MTHFR mutation you will need methylfolate as well since it is part of the methylation chemistry your body uses to activate B12 and folic acid, so they can work in your tissues properly. The mutation is a lifetime genetic thing.
If you use acid blocking drugs for GERD and heartburn, these drugs interfere with getting B12 from food.
Some people have a fast response to B12 supplementation, so you may see improvements before the 3 months are up.
Methylcobalamin is becoming more common in stores in the US. Walgreen's, Costco, Sam's club sell it now. But you may have to find it online depending on where you live. Amazon, iHerb, vitacost, Puritan's, Swanson's, all have it now. It remains inexpensive, and easy to fix.
Forget about the shots... use your doctor for testing only. Most doctors are not up to date at all on B12 issues.
Here is an example of a medical website for doctors that is easy to read and understand.
Vitamin B12 Deficiency - American Family Physician
This is a very good video. The woman who made it has a book out about it too. She does not go into the methylcobalamin information, on it though, but it is in her book. She is a nurse.
Diagnosing and Treating Vitamin B12 Deficiency - YouTube
This is from a long paper on PubMed, and has a table about the measurement in volunteers who took oral B12 and also had injections.
PubMed Central, Table 1: Blood. 28 Sep 15; 112(6): 2214–2221. Prepublished online 28 Jul 7. doi: 1.1182/blood-28-3-4253
How I Treat: How I treat cobalamin (vitamin B12) deficiency
The only major interaction I have found is that people who have the rare blood condition, called polycythema vera, should not supplement B12 unless supervised by a doctor. This blood condition is where too many red blood cells are made (a genetic error), and since B12 is involved in red blood cell manufacture by the bone marrow, these patients may see a rise in red blood cell counts.
Otherwise, B12 remains OTC (over the counter) for people to buy for themselves. Deficiency is very common today, and when it is severe then shots are used daily for a week or so until the patient rallies. I have seen this being done in nursing homes, where there is staff available to monitor and give the shots in the beginning.
But really, shots are not necessary for the most part, and remain an historical habit. New studies show oral works well, if done correctly.