The Methyl-cobalamin form is more easily absorbed into the system, rather than the more usual cyanocobalamin found in most B complex and multivitamins.
You get more help per mcg.
Like vit 'C', B12 can be take in mega doses of 1000mcg (1mg) to 5000mcg (5mg) or more, daily and excess is passed off into the urine.
I don't think it is possible to OD on methyl-cobalamin, sublingual.