Hi Slimlad, when i first got neuropathy i was very deficient in B12, but i did eat foods rich in B12, i believe that some people don't absorb the vitamins out of food as good as others for whatever reason, diseases like diabetes which can cause frequent urination which depletes the B vitamins quickly.
I have read that by Japanese PN experts standards, a serum b12 score of less than 500 is very suspicious of b12 deficiency, then they use mega doses of B12 methylcobalamin on their PN patients, up to 60 mg's daily.
B12 needs the other B vitamins to work with for max benefits, so i think it is wise to take a good quality Multi B be taken every day as well.
A B12 Methylcobalamin supplement is a large molecure and should be taken on an empty stomache for maximum absorption [ so the stomache has not got anything else to contend with ] at least 1mg or up to 5 mg daily, injectons of b12 hydroxocobalamin are good too, b12 Cynocobalamin supplementation is ok but it has to be converted by the liver to get the active form [methylcobalamin] so it takes longer to build up good storage.
B12 is water soluble, so what the body doesn't need it will urinate out the rest, you can not overdose on b12, it is not dangerous at all, nerves are reliant on the B vitamins for nourishment, B12 it is not a cure all for all PNer's because of the huge amount of different reasons why the nerves get damaged, lack of b12 is only one reason out of the many.