Low/deficient vitamin B12 is usually due to conditions that cause malabsorption unless you are a vegan/vegetarian.
Vitamin B12 sources include fish
and shellfish, meat (especially liver), poultry, eggs, milk, and milk products.
Excerpt from "Skin rashes from vitamin B12" by Ed Donner:
"Side effects of B12 supplements are rare, according to Southern Care Healthcare Group, and generally occur with the injections and not with oral supplements.