When you take it orally, only a small portion is absorbed from that 5000mcg/5mg tablet. You do not need intrinsic factor at all at this level of dose, because the B12 when given in high amounts is absorbed passively without it in the small intestine.

Shots only stay in the serum for 72 hrs. Taking orally provides the B12 in a more normal way (mimics getting from food), and then the B12 can get to more places more efficiently. The spinal cord fluid which is also in the brain, (called the CSF) takes the B12 from the serum by passive diffusion. When serum levels are low, less can get into the brain therefore. On that link I gave you is a paper from Dr. C. Snow, which explains neurological problems from "low B12". Since B12 is non-toxic and does not have negative effects, taking it in high doses is not problematic.

I raised my B12 levels to 1999 in 3 months on 5mg a day.
Many of our members at our Peripheral Neuropathy board and MS, MG boards take it this way. It is important to take on an empty stomach to facilitate the absorption, because food acts like a sponge to keep B12 away from the lining of the intestine. B12 is a very large molecule and has difficulty diffusing thru.

We are lucky that it is inexpensive and non-toxic so we can take control of providing it ourselves. So many doctors have no clue.
The medical information IS out there, doctors just don't look for it.

You should start to feel some improvement within 2 weeks. Some people feel energized, but I didn't so much. But since methyl B12 is the cofactor for melatonin synthesis in the brain, you should see improved sleeping when your levels become more normal. You will want to be high for a while over 1000 until you are better. Then you can choose to take 1000mcg/1mg daily thereafter. The liver stores B12 for the future, so each dose is providing that as well...takes a while to accumulate.