Who Should Not Take Vitamin B12
Vitamin B12 has a reputation as one of the safest supplements available. It is water-soluble, no tolerable upper intake level has been set in the EU, and high-dose tablets and sprays are sold freely on the high street. Yet "safe for most" is not the same as "right for everyone". Our editorial team sets out who should avoid B12 supplements, who needs medical supervision, and when taking B12 can hide a more important problem.
Why B12 is usually so well tolerated
Vitamin B12, or cobalamin, is needed for making red blood cells, maintaining the nervous system and producing DNA. It is found almost exclusively in animal foods such as meat, fish, eggs and dairy, which is why vegans, older adults and people with gut absorption problems are at the greatest risk of deficiency.
Absorption is tightly controlled. B12 from food binds to intrinsic factor, a protein produced in the stomach, and is then absorbed in the last part of the small intestine. This system saturates at small amounts per meal, so with high-dose tablets only around 1% is absorbed by simple diffusion.
The NHS advises that adults need about 1.5 micrograms a day, and states that taking 2 mg (2,000 micrograms) or less daily in supplements is unlikely to cause harm. The European Food Safety Authority has not established a tolerable upper intake level, because adverse effects at high intakes have not been demonstrated.
Supplements come in several forms: cyanocobalamin, methylcobalamin, hydroxocobalamin and adenosylcobalamin. In the UK, hydroxocobalamin injections are the standard prescribed treatment for pernicious anaemia and other forms of deficiency where absorption is impaired.
Absolute contraindications
The clearest reason not to take a B12 product is a known hypersensitivity to cobalamin or to any ingredient in the preparation. Allergic reactions are rare, but they are well documented, particularly with injections, and range from itching and rash to anaphylaxis. Anyone who has reacted before should not be re-exposed without specialist assessment.
Cobalt allergy deserves a mention. Cobalamin contains a cobalt atom at its centre, and some people with a confirmed cobalt allergy react to B12 preparations. They should discuss options with an allergist or dermatologist.
The other notable contraindication applies specifically to cyanocobalamin in people with Leber's hereditary optic neuropathy (LHON), a rare inherited eye disease. Cyanocobalamin has been associated with faster optic nerve damage in this condition, so UK product information lists it as a contraindication. Hydroxocobalamin is generally preferred for these patients, under specialist care.
People with tobacco amblyopia, a form of visual loss linked to heavy smoking and cyanide exposure, are similarly advised to avoid cyanocobalamin, because their ability to detoxify cyanide may be impaired.
| Group | Recommendation | Reason |
|---|---|---|
| Known allergy to B12 or cobalt | Avoid unless assessed by a specialist | Risk of allergic reactions, including anaphylaxis |
| Leber's hereditary optic neuropathy | Avoid cyanocobalamin | May accelerate optic nerve damage |
| Tobacco amblyopia | Avoid cyanocobalamin | Impaired cyanide handling |
| Diabetic kidney disease | Avoid high-dose B vitamins unless prescribed | Trial linked high doses to faster decline |
| Undiagnosed anaemia or neuropathy | Get tested before supplementing | Supplements can mask the underlying cause |
Conditions that call for caution
Kidney disease is the most important caution. In the DIVINe trial, published in JAMA in 2010, people with diabetic nephropathy given high doses of folic acid, vitamin B6 and vitamin B12 (as cyanocobalamin, 1 mg per day) experienced a faster decline in kidney function and more cardiovascular events than those on placebo. The cyanide released from cyanocobalamin, which reduced kidneys clear less efficiently, was proposed as one possible explanation.
People with severe megaloblastic anaemia who start B12 treatment can develop low potassium levels as rapid red cell production pulls potassium into new cells. This is why treatment of established deficiency should begin under medical supervision, with blood tests, rather than with self-bought tablets.
Polycythaemia vera, a bone marrow disorder that causes too many red blood cells, is another situation where unsupervised B12 is unwise, because correcting a coexisting deficiency can unmask the full extent of the disorder.
Acne and rosacea flares have been reported with high-dose B12, particularly injections. Research has suggested that B12 may alter the activity of skin bacteria. People prone to acne who notice breakouts after starting a supplement may wish to reduce the dose or stop.
When self-treating masks a problem
Perhaps the most common real-world risk is not toxicity but delay in diagnosis. Fatigue, pins and needles, a sore tongue, memory problems and low mood can all be symptoms of B12 deficiency, but they can also stem from many other conditions. Taking B12 without a diagnosis can blur the picture.
If you start supplements before a blood test, your B12 level may appear normal even though the underlying cause, such as pernicious anaemia, coeliac disease or a medication effect, remains untreated. Pernicious anaemia is an autoimmune condition that needs lifelong management, typically with injections.
The relationship between B12 and folate is also important. High-dose folic acid can correct the anaemia of B12 deficiency while allowing nerve damage to progress. Conversely, isolated B12 supplementation will not fix a folate deficiency. Both should be tested together.
Before starting a B12 supplement for symptoms, it is therefore sensible to:
- see your GP and describe all symptoms, including any numbness or balance problems;
- ask for a full blood count with B12 and folate levels;
- mention any long-term medicines, diet changes or digestive problems;
- only then start supplements, or receive injections if absorption is impaired.
Medicines, high levels and practical advice
Several common medicines reduce B12 absorption, so people taking them are more likely to need B12 than to avoid it. These include metformin, proton pump inhibitors such as omeprazole, H2 blockers, and colchicine. Nitrous oxide, including recreational use, inactivates B12 and can cause serious neurological damage.
Some medicines and conditions can interfere with response to treatment. Chloramphenicol, for example, may blunt the blood response to B12. Long-term heavy alcohol use also impairs absorption and storage.
A high blood B12 level in someone not taking supplements is a different matter. It can be associated with liver disease, kidney disease and certain blood disorders, so an unexplained raised result should be investigated rather than ignored. It is not, in itself, a reason to stop eating B12-containing foods.
For healthy adults, including athletes and vegans, a supplement in line with NHS guidance is reasonable. There is no evidence that extra B12 improves sports performance in people who are not deficient, despite its popularity as an "energy" injection at private clinics.
Editorial conclusions
Vitamin B12 is among the safest nutrients for the vast majority of people, and deficiency is far more common than harm from excess.
Clear exceptions do exist: allergy to cobalamin or cobalt, Leber's hereditary optic neuropathy and tobacco amblyopia (specifically with cyanocobalamin), and diabetic kidney disease when high doses are combined with other B vitamins.
The greater everyday risk is self-treating vague symptoms without a diagnosis, which can mask pernicious anaemia or folate deficiency. A blood test first, then targeted treatment, is the safest order.
If you found this useful, we also suggest our articles on B12 for vegan athletes, on blood tests while taking metformin, and on the differences between methylcobalamin and cyanocobalamin.
References
- Stabler SP. Vitamin B12 deficiency. N Engl J Med. 2013;368(2):149–160.
- House AA, Eliasziw M, Cattran DC, et al. Effect of B-vitamin therapy on progression of diabetic nephropathy: a randomized controlled trial. JAMA. 2010;303(16):1603–1609.
- Devalia V, Hamilton MS, Molloy AM; British Committee for Standards in Haematology. Guidelines for the diagnosis and treatment of cobalamin and folate disorders. Br J Haematol. 2014;166(4):496–513.
- Kang D, Shi B, Erfe MC, Craft N, Li H. Vitamin B12 modulates the transcriptome of the skin microbiota in acne pathogenesis. Sci Transl Med. 2015;7(293):293ra103.
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on dietary reference values for cobalamin (vitamin B12). EFSA J. 2015;13(7):4150.
- Green R, Allen LH, Bjørke-Monsen AL, et al. Vitamin B12 deficiency. Nat Rev Dis Primers. 2017;3:17040.
