Updated June 2026 · Ireland Health Shop · The Honey Pot, Clonmel
Vitamin B12 (cobalamin) is a water-soluble B vitamin with a uniquely complex molecular structure — it's the largest and most structurally complex vitamin known. It contains cobalt at its core, which gives it its alternative name. Unlike most water-soluble vitamins, the body stores B12 in the liver — adults typically hold 2–5 years' worth of supply. This means deficiency develops slowly and can be years in the making before symptoms appear.
B12 is essential for the conversion of carbohydrates to glucose — the primary fuel for brain and body. It's a co-factor in the production of succinyl-CoA, which feeds the citric acid cycle (energy production). This is why B12 deficiency is so closely associated with fatigue — cells literally cannot produce energy efficiently.
B12 is required for the maturation of red blood cells in the bone marrow. Deficiency causes megaloblastic anaemia — large, abnormal red blood cells that can't function properly and can't carry oxygen efficiently. Symptoms include profound fatigue, pallor, breathlessness, and palpitations. This type of anaemia looks different from iron-deficiency anaemia on a blood test (MCV is elevated rather than low).
B12 is critical for maintaining the myelin sheath — the protective coating around nerve fibres. Without adequate B12, myelin degrades, causing a condition called subacute combined degeneration of the spinal cord. Neurological B12 deficiency causes: tingling and numbness in the hands and feet, weakness, poor coordination, difficulty walking, cognitive impairment, and psychiatric symptoms including depression and paranoia. Crucially, neurological damage from B12 deficiency can be permanent if not caught and treated early.
B12 works alongside folate (B9) in the methylation cycle, which is essential for DNA synthesis and repair. This is why B12 deficiency during pregnancy can cause neural tube defects (though folate deficiency is more commonly implicated in this).
B12 (along with B6 and folate) is required to convert homocysteine — a potentially toxic amino acid — into methionine. Elevated homocysteine is a significant independent risk factor for cardiovascular disease, stroke, and dementia. B12 deficiency allows homocysteine to accumulate, directly increasing cardiovascular and cognitive risk.
B12 occurs almost exclusively in animal products — meat, fish, eggs, and dairy. There is no reliable plant-based source of bioavailable B12 (despite claims about certain algae or fermented foods). Vegans who do not supplement or eat fortified foods will develop B12 deficiency — it's a matter of when, not if. This is not a controversial statement; it's the consensus of every major nutrition authority including the HSE, the BDA, and the Academy of Nutrition and Dietetics. If you follow a vegan diet and do not supplement B12, please start today.
Vegetarians who eat dairy and eggs have some dietary B12 intake, but studies consistently show lower B12 status in vegetarians compared to omnivores. The bioavailability of B12 from dairy is lower than from meat/fish. Many vegetarians are marginal rather than overtly deficient, but supplementation is still sensible.
Gastric acid production declines with age. B12 from food must be cleaved from food proteins by stomach acid before it can be absorbed — reduced acid means reduced liberation of B12 from food. Additionally, intrinsic factor (a protein produced by the stomach, essential for B12 absorption in the small intestine) production decreases with age. For these reasons, many older adults become B12 deficient despite adequate dietary intake. The US Institute of Medicine recommends that people over 50 get most of their B12 from fortified foods or supplements (where it's free — not protein-bound and doesn't require gastric acid).
Pernicious anaemia is an autoimmune condition where the immune system destroys the parietal cells of the stomach that produce intrinsic factor. Without intrinsic factor, dietary B12 absorption is completely blocked. This is the most common cause of severe B12 deficiency in Ireland and usually requires treatment with B12 injections (hydroxocobalamin), as oral supplements alone may not be absorbed adequately. It affects approximately 1 in 1,000 people, with higher prevalence in those of Irish and Northern European descent.
Metformin — the most commonly prescribed diabetes medication in Ireland — reduces B12 absorption by interfering with calcium-dependent uptake in the ileum. Long-term metformin users should have B12 levels monitored annually by their GP.
Proton pump inhibitors (omeprazole, pantoprazole, esomeprazole) and H2 blockers (famotidine, ranitidine) dramatically reduce stomach acid production. With less acid available to free B12 from food proteins, long-term users are at substantial risk of deficiency.
B12 deficiency can mimic many other conditions, making it easy to miss:
If you're experiencing several of these symptoms — especially if you're vegan, vegetarian, or over 50 — ask your GP for a serum B12 test. It's a simple blood test.
This is one of the most common questions at The Honey Pot's supplement counter.
The synthetic form — most widely used in fortified foods and pharmaceutical-grade supplements. Contains a cyanide molecule (at concentrations far too low to be toxic for healthy people). Must be converted by the body into methylcobalamin or adenosylcobalamin (the two active forms). Has the most clinical research behind it, is extremely stable, and is highly effective at raising blood B12 levels in most people. This is the form used in NHS/HSE injections for pernicious anaemia.
One of the two active co-enzyme forms of B12 — it can be used directly without conversion. Some practitioners argue it is better retained in tissue. Available evidence suggests both forms are effective at correcting deficiency; methylcobalamin may be preferred for neurological conditions and for people with MTHFR gene variants who have impaired methylation. Some people feel more energetic on methylcobalamin, though this is difficult to establish rigorously.
The other active co-enzyme form. Used in mitochondrial energy metabolism. Less commonly available in supplements.
The form used in B12 injections for pernicious anaemia in Irish hospitals and GP practices. Longer-acting than cyanocobalamin. Also available as nasal spray or sublingual drops in some markets.
Bottom line: For most people, either cyanocobalamin or methylcobalamin will correct B12 deficiency effectively. If you have confirmed neurological symptoms, MTHFR variants, or just prefer a more "natural" active form, methylcobalamin is a reasonable preference.
B12 is absorbed in the small intestine via a specific receptor that requires intrinsic factor — and this receptor can only process a limited amount at a time (roughly 1.5 micrograms per dose). This means that at standard doses, there's a ceiling on absorption. However, at very high doses (1,000 micrograms and above), approximately 1% can be absorbed passively without intrinsic factor — which is why high-dose oral B12 is an effective alternative to injections even in pernicious anaemia for some patients.
Sublingual (under-the-tongue) sprays and lozenges are a popular option as they allow direct absorption through the oral mucosa, bypassing gastric acid — useful for people with absorption issues. They're well-tolerated and convenient.
Find B12 supplements — including sublingual methylcobalamin — at The Honey Pot, Clonmel
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