I have been hospitalized due to a B12 level that was so low it couldn't be measured. For the past year I've been on injections, 1000mcg, every 3 weeks. My B12 level remained at ~250 (range 198-982). Then I read this article ncbi.nlm.nih.gov/pmc/articl...
For the past 3 months I've been using sublingual B12 tablets. I started with 1000mcg/day, while continuing with my injections. My B12 level rose to 458. For the past 3 weeks I increased my B12 tablet level to 2000mcg/day, again keeping the every 3 week injections as well. My B12 level is now 528. I'm considering eliminating the injections and going with the sublingual tablets entirely.
I live in the US and don't have the issues that I'm reading about on this blog in dealing with the NHS. Read the article and try the tablets. In the US they are cheap, $9.00 for a 100 at the 1000mcg level and $15.00 at the 2500 mcg level. They are also available at a 5000mcg level.
I hope this helps.