If a person is hugely deficient and near death, a series of daily injections of cyano is required to prevent death. I see this in nursing homes frequently. By the time patients get to long term care they are very very ill...near death.

For those patients living a life...walking/talking breathing, etc, oral B12 (not necessarily sublingual) will do as well. Sublingual claims are not sterling. Most of what dissolves in your mouth is swallowed with your saliva.

Take at least 1000mcg (1mg) daily on an empty stomach, and you'll do as well as injectable. Studies have confirmed this.
If you are very low, you can do 5mg daily for a month or two...and decrease to 1mg daily after that. Don't assume you will be ever off it...it is most likely a lifetime thing. Whatever has caused your low readings is not going to change.