Who Should Not Take B12 Supplements?

October 7, 2025 •

3 min read

While vitamin B12 is essential for nerve function, DNA production, and red blood cell formation, certain individuals should not take B12 supplements due to specific health risks. For example, those with Leber's hereditary optic neuropathy may experience worsened vision loss if they supplement with cyanocobalamin, a common form of B12.

Quick Summary

This article outlines specific medical conditions and circumstances where taking vitamin B12 supplements can be harmful, covering allergies, genetic disorders, kidney problems, and potential interactions with other treatments. Certain health conditions, allergies, and the use of specific medications necessitate caution or avoidance of B12 supplementation.

Key Points

Leber's Disease: Those with Leber's hereditary optic neuropathy should avoid cyanocobalamin, a type of B12, to prevent potential vision damage.
Allergies: Individuals with known hypersensitivity to cobalt or B12 should not take supplements to avoid severe allergic reactions like anaphylaxis.
Kidney Disease: Patients with renal impairment should consult a doctor, as certain B12 formulations contain aluminum that can accumulate and become toxic.
Polycythemia Vera: People with this blood disorder should avoid B12, as it can worsen their condition by increasing red blood cell production.
Drug Interactions: Certain medications, including metformin and stomach acid reducers, can interfere with B12 absorption and require medical supervision.
Folic Acid Mismanagement: Taking high-dose folic acid alone can mask a B12 deficiency, potentially leading to permanent neurological damage.
Diagnosis Before Supplementation: Medical diagnosis is essential to determine if B12 is needed, especially in complex cases involving kidney disease or malabsorption syndromes.

Critical Conditions and Allergies Requiring Caution

Individuals with certain pre-existing health conditions or allergies should be very cautious or completely avoid taking vitamin B12 supplements. A known allergy or hypersensitivity to vitamin B12 or cobalt is a firm contraindication, as it can lead to severe allergic reactions, including anaphylaxis. This life-threatening reaction requires immediate medical attention and can present with symptoms such as difficulty breathing, rapid heartbeat, and swelling of the face, tongue, or throat. Patients with a history of any such sensitivity must inform their healthcare provider before considering B12 supplementation.

Leber's Hereditary Optic Neuropathy (LHON)

One of the most critical contraindications is Leber's hereditary optic neuropathy, a mitochondrial disorder causing progressive vision loss. In individuals with LHON, supplementing with cyanocobalamin, a common synthetic form of B12, can potentially accelerate or exacerbate optic nerve damage, leading to irreversible blindness. For carriers of the LHON gene mutation, a B12 deficiency can even trigger the onset of vision loss. Therefore, both symptomatic patients and asymptomatic carriers should be carefully monitored and avoid cyanocobalamin.

Polycythemia Vera

People with polycythemia vera, a rare blood cancer that causes the body to produce too many red blood cells, should also avoid B12 supplements. B12 supplementation can further stimulate red blood cell production, worsening the underlying condition. This can exacerbate symptoms and increase the risk of complications associated with excessive red blood cell count, such as blood clots.

Kidney Disease

Caution is advised for individuals with chronic kidney disease (CKD), particularly those with advanced disease or on dialysis. While B12 deficiency can be common in dialysis patients, standard B12 supplements formulated with cyanocobalamin may be problematic. Cyanocobalamin contains small amounts of aluminum, and with impaired kidney function, this can lead to aluminum accumulation, potentially causing central nervous system or bone toxicity. Renal impairment also raises levels of homocysteine and methylmalonic acid, which complicate B12 deficiency diagnosis. Patients with CKD should only take B12 supplements under strict medical supervision and ideally use a form other than cyanocobalamin.

Potential Drug and Nutrient Interactions

Certain medications can affect B12 absorption or metabolism, necessitating a conversation with a doctor before starting a supplement.

Metformin: Long-term use of this common diabetes medication can interfere with B12 absorption. Patients on metformin should be monitored for B12 levels and may require supplementation, but a healthcare provider should manage this process.
Chloramphenicol: This antibiotic can suppress bone marrow activity and may interfere with the body's response to B12 supplementation.
Acid-Reducing Medications: Proton pump inhibitors (e.g., omeprazole) and histamine H2 receptor antagonists (e.g., ranitidine) reduce stomach acid, which is necessary for releasing B12 from food for absorption. Long-term use can lead to B12 deficiency.
High-Dose Folic Acid: Taking high doses of folic acid can mask the symptoms of an underlying B12 deficiency, potentially leading to irreversible nerve damage if the B12 deficiency is not also addressed.

Comparison of Supplementation Scenarios


Feature	B12 Supplementation for the General Population	B12 Supplementation for Individuals with Contraindications
Need for Supplement	Generally not needed if consuming animal products; beneficial for vegans/vegetarians.	Necessary for treating diagnosed deficiency but requires careful medical management.
Safety Profile	Considered safe, as excess is water-soluble and excreted.	High risk of adverse effects, severe allergic reactions, or exacerbation of underlying conditions.
Monitoring	Minimal to no monitoring required for healthy individuals.	Close medical supervision and regular lab tests are crucial.
Appropriate Form	Oral supplements and diet are typically sufficient.	Injection may be required for malabsorption; specific B12 forms (e.g., methylcobalamin) may be preferred over cyanocobalamin.
Risk of Complications	Low to none.	High, including nerve damage in LHON, and increased red blood cell count in polycythemia vera.

Conclusion: Navigating B12 Supplementation Wisely

Although vitamin B12 is a vital nutrient, it is not universally safe for everyone. People with specific conditions, such as Leber's hereditary optic neuropathy, polycythemia vera, and advanced kidney disease, must avoid or carefully manage their intake. Those with a confirmed allergy to B12 or cobalt, or who are on medications that interfere with B12 absorption, should also exercise extreme caution. It is crucial for anyone with these health issues to consult a healthcare professional before starting any B12 supplement to ensure their safety and avoid severe complications. Do not self-diagnose or self-prescribe, as a doctor can determine the appropriate course of action based on individual needs. For more information, you can review the FDA drug label warnings.

Frequently Asked Questions

If you have kidney disease, especially with impaired function or on dialysis, you should consult a doctor before taking B12. Some B12 supplements contain aluminum, which can accumulate and cause toxicity in individuals with poor kidney function.

For individuals with Leber's hereditary optic neuropathy (LHON), supplementing with cyanocobalamin (a common form of B12) can exacerbate optic nerve damage and may lead to blindness. A B12 deficiency can also trigger vision loss in carriers, so medical management is crucial.

Since B12 contains cobalt, an individual with a cobalt allergy who takes B12 could experience a severe allergic reaction, including life-threatening anaphylaxis. Symptoms include difficulty breathing, hives, and swelling.

Yes. Taking high-dose folic acid can mask the symptoms of a B12 deficiency, potentially allowing neurological damage to progress undetected. A doctor must manage both deficiencies simultaneously.

People taking metformin for diabetes, stomach acid reducers, or certain antibiotics like chloramphenicol are at risk for drug interactions that can affect B12 levels or absorption. Medical consultation is necessary to manage these situations.

Yes, B12 supplements can worsen polycythemia vera, a condition of excessive red blood cell production. Since B12 stimulates red blood cell formation, it can exacerbate the underlying disorder.

While B12 is water-soluble and generally considered safe at high doses, excessively high levels may indicate an underlying medical condition. Though rare, some side effects like skin changes (acne, rashes) or headaches have been reported with very high intake.