Make sure you are getting methylcobalamin. There are, in my opinion, unscrupulous marketers jumping on the "sublingual" bandwagon and selling "sublingual" cyanocobalamin for a lot of money. What a crock.
It's been about 40 years now since researchers learned that the large dose 1000 - 2000 mcg (all at once) on treats B12 malabsorption as well as shots. Since then it has been shown that in many if not most cases lab results show even better results with oral. And all that is with swallowed, relatively inefficient, cyanocobalamin.
So just make sure you don't pay a bunch of money for someone's hype.
There are other good brands, but the one I have seen lab results on over the years is Jarrow. Some are pharmaceutical grade, do well consistently when tested independently, etc. Beware of a company that doesn't make the ingredients easy to find or makes it sound like they've got something no one else has.
1000 - 5000 mcg methylcobalamin is the most common range. I think the 5000mcg is a better value, and a person can never be sure that the larger dose won't help more, and after really loading up for a good long while, one might cut back to one every two or three days. Safe and relatively inexpensive, so.....