I realise this is an old thread but would like to add my tuppence.
B12 deficiency is not as simple as ’just‘ getting a blood test & being treated - or not.
Unfortunately, the serum blood test only measures the total in the blood. It does not determine how much is ‘active’ or ‘inactive’. There are two inactive forms, Cyanocobalamin & Hydroxocobalamin & two active forms, Methylcobalamin & Adenosylcobalamin. Only the active forms work at cellular level.
If your body can convert the inactive to the active, you are ok as long as you are getting enough in your diet & can absorb it digestively. If it can’t convert it very well, it doesn’t matter how much B12 you get, most of what is in your bloodstream will be inactive & unavailable, despite the numbers looking good on paper. Even those who get IV shots - if they are as inactive forms - may still struggle.
Because doctors put far more faith in test results than in symptoms, if they deem the test results to indicate that you have ‘adequate’ B12 in your blood (regardless of whether you can actually use it or not), then B12 deficiency will be dismissed and attention will be directed at everything but the real issue.
Dr. Joseph Chandy was mentioned in a previous comment. He noticed the same symptoms in many of his patients as he had witnessed in Vegans he treated in the Punjab prior to moving to the U.K. Hooray for that man! Here was a genuine, old-school doctor who knew & understood the importance of knowing & recognising symptoms! He found that at least 50% of his patients were actually B12 deficient!
if you translate that to the Western population, that is a huge amount of people! Between grossly ignored B12 & Thiamine (B1) deficiencies (& probably other B vitamins too, as they are all important & support each other), there is another very insidious pandemic going on.
Why should this be such an issue? I don’t know, but I have my suspicions that the use of antibiotics over the last 70 years may well have decimated & destroyed our B vitamin-producing & metabolising gut microbe populations, so many of us are unable to utilise them effectively.
Personally, I use sublingual liquid B12 drops with BOTH active forms & they work very well & very quickly - better than any other types I’ve tried. So many people have gut & digestive issues these days, metabolic syndrome, etc, & take B12-inhibiting medications, & have other issues that affect absorption & utilisation, it is actually a much bigger issue than is recognised.