If you are referring to high levels in the serum, remember that serum levels fluctuate greatly from day to day and with supplementation. Sometimes serum levels show up high with supplementation. This does not mean a toxicity. B-12 levels are best measured in the cell itself. Not to say that it isn’t possible, but I have not seen a single case of high B-12 levels in 12 years of practice.
The following video goes into more detail on vitamin B-12:
All the best,