NHS Diagnosis: Vitamin B12 or folate deficiency anaemia In particular, this paper explains that A particular drawback of testing vitamin B12 levels is that the current widely used blood test only measures the total amount of vitamin B12 in the blood This means it measures forms of vitamin B12 that are active and can be used by the body, as well as the bound or inactive forms, which cannot be used by the body If a significant amount of the vitamin B12 in the patients blood is inactive, a blood test may show normal B12 levels, even though the patient cannot use much of it and might have Vitamin B12 deficiency This is why its important for symptoms to be taken into account when a diagnosis is made

How Vitamin B12 Affects Brain Chemistry and Stress Systems Neurotransmitter production B12, together with folate and B6, is essential for the methylation reactions that produce serotonin, dopamine, norepinephrine, and GABA the neurotransmitters most directly involved in regulating mood, motivation, calm, and emotional stability
I will continue using it
But in reality, thats often not the case