Is there anyone who should not take vitamin B12?

October 29, 2025 •

3 min read

While Vitamin B12 is crucial for nerve function and red blood cell production, there are specific, critical situations where certain individuals should not take vitamin B12 or require strict medical supervision. Leber's disease, for instance, is a rare but vital contraindication that merits special attention.

Quick Summary

Some individuals, including those with Leber's disease, cobalt/B12 allergies, or polycythemia vera, should avoid B12 supplements. Chronic conditions or certain medications also require medical consultation.

Key Points

Leber's Disease: People with this rare genetic eye condition should strictly avoid vitamin B12 due to the risk of worsening optic nerve damage.
Allergies: A known allergy to cobalt, a component of vitamin B12, or hypersensitivity to B12 itself is a critical contraindication.
Polycythemia Vera: This blood cancer is exacerbated by B12 supplementation, which can dangerously increase red blood cell production.
Medical Supervision: Individuals with kidney or liver disease should only take B12 under a doctor's supervision due to metabolic and excretion concerns.
Medication Interference: Certain drugs, including Metformin and stomach acid inhibitors, can affect B12 levels and absorption, requiring a medical review of supplementation.
Post-Stent Care: Avoid B-vitamin combinations after coronary stent placement due to a potential risk of blood vessel narrowing.

Critical Contraindications for Vitamin B12

While vitamin B12 is often considered safe, there are several key medical conditions where supplementation is contraindicated and potentially dangerous.

Leber's Hereditary Optic Neuropathy (LHON)

Individuals with Leber's disease or a family history of this genetic vision loss condition should strictly avoid vitamin B12, particularly the cyanocobalamin form, as it can worsen optic nerve damage. Medical history is crucial to identify this risk before supplementation.

Cobalt and Vitamin B12 Hypersensitivity

Vitamin B12 contains cobalt, and those with a cobalt allergy risk severe reactions, including anaphylaxis, from B12 supplements, especially injections. Allergic reactions to B12 itself or other ingredients are also possible.

Polycythemia Vera

Patients with polycythemia vera, a blood cancer characterized by excessive red blood cell production, should avoid B12 supplements. As B12 aids red blood cell creation, supplementation could exacerbate the condition and increase the risk of blood clots.

Conditions Requiring Medical Supervision

Certain health issues require medical oversight before starting B12.

Kidney or Liver Disease

Individuals with impaired kidney or liver function may have difficulty processing and eliminating B12. High B12 levels might signal underlying organ issues rather than just supplementation, requiring a doctor's evaluation and monitoring.

After Coronary Stent Placement

High doses of B vitamins (B12, folate, B6) have been associated with increased blood vessel narrowing after coronary stent procedures. Patients should avoid high-dose B12 unless advised by a cardiologist.

Potential Risks and Medication Interactions

High-Dose Supplementation and Health Risks

While B12 has low toxicity, some research suggests a link between very high B12 levels and risks like certain cancers, though findings are inconsistent. Avoid megadoses without a diagnosed deficiency and medical guidance.

Medications That Affect Vitamin B12 Levels

Some medications interfere with B12 absorption, potentially leading to deficiency. Supplementation may be needed but requires careful management.

Common Medications Affecting B12 Absorption:

Metformin: Can reduce B12 absorption.
Gastric Acid Inhibitors (PPIs, H2 blockers): Reduce stomach acid needed for B12 release.
Chloramphenicol: May interfere with B12 effects.
Colchicine: Can impair B12 absorption.

Comparison of Vitamin B12 Risk Groups


Risk Category	Specific Condition(s)	Recommendation	Reason for Caution
Critical Contraindication	Leber's Hereditary Optic Neuropathy (LHON), Cobalt/B12 Allergy, Polycythemia Vera	Do NOT take B12 supplements.	Potentially worsens optic nerve damage, risk of severe allergic reaction (anaphylaxis), or exacerbates blood cancer.
Requires Medical Guidance	Kidney/Liver Disease, Post-Coronary Stent Placement, Certain Cancers	Consult a doctor for dosage and monitoring.	Impaired metabolism, potential for increased health risks, or underlying illness indicated by high B12.
Requires Absorption Management	Long-term use of Metformin, PPIs, H2 blockers, Colchicine	Talk to a doctor about supplementation strategy.	Medications interfere with absorption, necessitating monitoring and often, supplementation.

Conclusion

While generally safe, vitamin B12 is not suitable for everyone. Individuals with Leber's disease, cobalt or B12 allergies, or polycythemia vera should avoid it. Those with kidney disease, liver disease, or a history of coronary stents need medical advice before supplementing. Always consult a healthcare provider to ensure B12 supplementation is safe and appropriate for your health needs.

For More Information

For those interested in the details of how B12 deficiency can manifest in rare and atypical ways, a case study highlighting isolated thrombocytopenia may be insightful: https://pmc.ncbi.nlm.nih.gov/articles/PMC10519260/.

Frequently Asked Questions

Yes, while rare, allergic reactions to vitamin B12 or its cobalt component have been reported. Reactions can range from skin rashes to severe anaphylaxis, especially from injectable forms.

Leber's disease is a hereditary optic neuropathy that causes vision loss. In these patients, B12, particularly cyanocobalamin, can accelerate optic nerve damage.

Patients with kidney disease should consult their doctor. High B12 levels can sometimes indicate underlying kidney issues, and supplementation requires careful monitoring.

High B12 levels are generally considered low-toxicity, as the vitamin is water-soluble. However, extremely high levels have been associated with potential health risks, such as certain cancers, though the research is mixed.

Injectable B12 can carry a higher risk of severe allergic reactions, as injections may contain impurities or preservatives that can trigger a response. A single case report documented a patient who reacted to an injection but tolerated an oral form.

Medications such as Metformin, gastric acid inhibitors like PPIs, and colchicine can interfere with B12 absorption and metabolism. A doctor should be consulted to manage these interactions.

Not necessarily. High B12 levels can result from supplementation. However, if you are not supplementing, it could be a sign of an underlying medical condition, such as kidney or liver disease, and should be evaluated by a doctor.