I just looked over some of the multi's and b complexes I was taking, and they averaged 250 mcg. per dosage. Enough for most, but not for those with absorption/ conversion/ transport difficulties.
For some years, off and on, I did take a sublingual b complex with 800 mcg. of B12, but I usually swallowed it after a short time...didn't think it would make a difference.
~5yrs. ago, I started 5-10 mgs. daily of jarrow methylcobalamin. It was good back then,(not now), I responded well, after 6mnths. ( read alot about it) When I felt better, I reduced the dosage to 5mg, then down to 2500 mcg, then I would skip days, thinking I would be fine. ( paresthesias, brain fog, etc. returned) Brain fog was first, so I didn't notice the other symptoms as much, until I began to get strong electrical type shocks from nerves.
I've read that in Japan, the lowest serum B12 level is ~500. I've read of many people, including children, having severe symptoms even with elevated B12 levels,>1200. For instance, Dr. Neubrander, who treats kids with ASD,using subcutaneous injections of methylcobalamin, finds most of them have elevated B12.
One possible reason for this is that some of us have a transport problem. ( there are many transport proteins for B12) So, we end up having to rely on Passive Diffusion, or Passive Transport for absorption. We flood our bodies with it and absorb about 1-2 % of intake.
Now, I'm taking a minimum of 5mg. daily of Enzymatic Therapy brand methylcobalamin. I must admit it works best in my case, but I feel somewhat cornered because I haven't had such good results from other brands.
There are many folks here who respond well to hydroxocobalamin as well as methylcobalamin, so, it may take some trial and error, possibly genetics testing, etc.
I wish it could be easier.
P.S. Many experts believe that one should try to keep serum B12 levels above 1000, even higher, once supplementation begins. ( mine are >3000 pg/ml ) If you can find some symptoms, and observe them, it may help.