Vitamin B12 deficiency is more common than many people realise, and its consequences extend far beyond fatigue. While tiredness and weakness are well-known symptoms, B12 plays a critical role in your nervous system, and deficiency can cause neurological damage that may become irreversible if treatment is significantly delayed. Understanding the warning signs and knowing when to test is essential.
What Does B12 Do?
Vitamin B12 (cobalamin) is essential for three key processes in your body: red blood cell production, DNA synthesis, and the maintenance of your nervous system. It is required for the production of myelin, the protective sheath that surrounds your nerve fibres and allows electrical signals to travel efficiently.
When B12 levels fall too low, myelin production is impaired and nerve fibres may begin to degenerate. This process, known as demyelination, can affect the brain, spinal cord, and peripheral nerves.
Neurological Symptoms of B12 Deficiency
The neurological effects of B12 deficiency can appear before any changes show up on a full blood count. A study by Lindenbaum et al. (1988), one of the earliest to document this pattern, found that 28% of patients presenting with neuropsychiatric symptoms due to cobalamin deficiency had no anaemia or macrocytosis [1]. While this was a small study (141 patients) and nearly four decades old, its central finding has been consistently supported by subsequent clinical experience: you cannot rely on a normal blood count to rule out neurological B12 deficiency.
NICE guideline NG239 (2024) recommends investigating promptly when neurological symptoms are present, even if the full blood count is normal [2].
Neurological symptoms of B12 deficiency include:
Peripheral neuropathy: numbness, tingling, or pins and needles in the hands and feet
Balance and coordination problems: unsteadiness when walking, particularly in the dark
Cognitive changes: difficulty concentrating, memory problems, or confusion
Mood disturbance: depression, irritability, or personality changes
Visual disturbance: blurred vision, which in rare cases can progress to optic neuropathy
Muscle weakness, particularly in the legs
In severe, prolonged deficiency, damage to the dorsal columns of the spinal cord can cause a condition called subacute combined degeneration, which affects balance, sensation, and coordination. Early treatment can halt and often reverse this damage, but significant delays carry a risk of permanent neurological impairment.
Who Is at Risk?
Certain groups are at significantly higher risk of B12 deficiency:
People following a vegan or strict vegetarian diet: B12 is found almost exclusively in animal products
Adults over 65: absorption declines with age due to reduced gastric acid production
People with autoimmune gastritis (previously called pernicious anaemia): an autoimmune condition that destroys the cells in the stomach lining responsible for producing intrinsic factor, which is required for B12 absorption
People taking long-term metformin, proton pump inhibitors (PPIs), or H2 receptor antagonists: these medications can impair B12 absorption
People with gastrointestinal conditions affecting the terminal ileum (where B12 is absorbed), including Crohn's disease, coeliac disease, or those who have had bowel surgery
Understanding Your B12 Results
A standard B12 blood test measures total serum B12. Under NICE guideline NG239 (2024), the diagnostic thresholds are [2]:
Below 133 pmol/L (180 ng/L): confirmed B12 deficiency
133 to 258 pmol/L (180 to 350 ng/L): indeterminate, possible deficiency requiring further investigation
Above 258 pmol/L (350 ng/L): deficiency unlikely
A normal serum B12 does not completely exclude functional deficiency. If your total B12 falls in the indeterminate range or your symptoms are strongly suggestive despite a "normal" result, further testing with methylmalonic acid (MMA) and homocysteine can help clarify the picture. These downstream metabolites accumulate when B12 is functionally insufficient, even if the total serum level appears adequate [3].
Active B12 (holotranscobalamin) is an alternative first-line test that measures only the biologically available fraction of B12. NICE NG239 accepts either total B12 or active B12 as the initial investigation, with active B12 deficiency defined as below 25 pmol/L and the indeterminate range spanning 25 to 70 pmol/L [2].
Folate should also be checked alongside B12. Both vitamins work together in the same metabolic pathway, and folate deficiency can mask B12 deficiency by preventing the expected rise in MCV (mean cell volume) that would otherwise alert your GP to the problem.
When to See Your GP
You should see your GP if you experience any neurological symptoms that could indicate B12 deficiency, including tingling, numbness, balance problems, or cognitive changes. Prompt investigation is important because neurological damage from B12 deficiency carries a risk of becoming irreversible if treatment is significantly delayed.
If your Brooksby results show a total B12 below 133 pmol/L or an active B12 below 25 pmol/L, take your report to your GP. They can arrange confirmatory testing and initiate appropriate prescription replacement therapy. If your result falls in the indeterminate range and you have symptoms, your GP can request MMA testing to determine whether functional deficiency is present.
How Brooksby Medical Can Help
Brooksby Medical's Nutrients and Energy Profile includes active B12 alongside ferritin, folate, and vitamin D, covering the most common nutritional causes of fatigue and neurological symptoms. Every result comes with a report from a practising GP that interprets your level in clinical context.
For a broader assessment, the Wellness Profile adds thyroid function, liver and kidney markers, and inflammation markers to the nutritional panel, giving you a more complete picture of your overall health.
References
Lindenbaum J, Healton EB, Savage DG, et al. Neuropsychiatric disorders caused by cobalamin deficiency in the absence of anaemia or macrocytosis. N Engl J Med. 1988;318(26):1720-1728
NICE. Vitamin B12 deficiency in over 16s: diagnosis and management (NG239). 2024. NICE.org.uk
Hannibal L, Lysne V, Bjørke-Monsen AL, et al. Biomarkers and algorithms for the diagnosis of vitamin B12 deficiency. Front Mol Biosci. 2016;3:27
Written by Dr James Coleman, GP and founder of Brooksby Medical. Dr Coleman is a practising General Practitioner who founded Brooksby Medical to give patients direct access to the blood tests and clinical interpretation they need, without waiting lists.
Medically reviewed: March 2026 | Next review due: March 2027
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Blood test results should always be interpreted by a qualified healthcare professional in the context of your individual symptoms, medical history, and clinical picture. If you have concerns about your health, please consult your GP.
Medical disclaimer. This article is for informational purposes and does not constitute medical advice. Blood test results should always be interpreted by a qualified healthcare professional in the context of your individual symptoms, history, and clinical picture. If you have concerns about your health, please consult your GP.