When Should You Not Take Vitamin B12?

October 6, 2025 •

3 min read

According to the NIH Office of Dietary Supplements, while Vitamin B12 is generally considered safe, there are specific medical conditions, drug interactions, and allergies that can make supplementation dangerous. Knowing when you should not take vitamin B12 is crucial for preventing serious side effects and potential complications.

Quick Summary

This article outlines critical scenarios where vitamin B12 supplementation is not recommended, focusing on medical conditions, known allergies, and specific medications that interact negatively with B12, along with potential side effects.

Key Points

Leber's Disease Risk: Avoid vitamin B12 if you have Leber's disease, as it can worsen the condition and lead to optic nerve damage.
Allergies to B12 or Cobalt: Do not take B12 supplements if you are allergic or sensitive to cobalt or cyanocobalamin, which can trigger severe allergic reactions.
Certain Blood Disorders: Patients with polycythemia vera should not take B12, as it can exacerbate the overproduction of red blood cells.
Drug Interactions: Exercise caution with B12 if taking metformin, gastric acid inhibitors (PPIs, H2 blockers), or other medications that interfere with absorption.
High Folic Acid Intake: Avoid high-dose folic acid alone if a B12 deficiency is suspected, as it can mask neurological damage.
Undiagnosed Deficiency: Always have a deficiency diagnosed by a doctor to determine the root cause, which could be more serious than a simple dietary lack.
Kidney or Liver Disease: Individuals with significant kidney or liver issues should consult a doctor before supplementing, as these conditions affect B12 metabolism.

Understanding the Contraindications and Risks of Vitamin B12

Vitamin B12, or cobalamin, is an essential water-soluble vitamin vital for nerve function, red blood cell formation, and DNA synthesis. While supplementation is common, especially for individuals with a deficiency due to diet or absorption issues, it is not safe for everyone. In certain situations, taking vitamin B12 can pose significant health risks, making it important to consult a healthcare provider before beginning a new regimen.

Conditions Where You Should Avoid Vitamin B12

For some individuals, taking vitamin B12 is actively discouraged due to pre-existing health issues. The following medical conditions represent clear contraindications where supplementation should be avoided unless specifically directed by a physician:

Leber's Disease: This hereditary optic nerve atrophy is a primary contraindication for vitamin B12 supplementation. The vitamin can exacerbate this condition, potentially leading to optic nerve damage and blindness.
Allergy or Hypersensitivity: Individuals with a known allergy to cyanocobalamin, cobalt, or any component of the supplement should avoid B12. Those with a cobalt sensitivity are at risk of an allergic reaction.
Polycythemia Vera: This rare blood disorder causes the bone marrow to produce too many red blood cells. B12 supplementation can further increase red blood cell production, worsening the condition.
Hypokalemia: High doses of B12, especially via injection, can stimulate the production of new blood cells, which consumes potassium. This can lead to low potassium levels.
Advanced Kidney or Liver Disease: These conditions can interfere with how the body metabolizes and stores B12, making high doses ill-advised without medical clearance.
Early-stage B12 deficiency being treated with folic acid only: Taking large amounts of folic acid can mask the symptoms of a B12 deficiency and allow irreversible nervous system damage to progress.

Comparing Risks of Oral vs. Injectable B12


Feature	Oral Vitamin B12 (e.g., Tablets, Lozenges)	Injectable Vitamin B12 (e.g., Shots)
Allergic Reactions	Less common, but possible with sensitivity to components.	Higher risk of severe allergic reactions (anaphylaxis), particularly due to cobalt or other ingredients.
Dosage Control	Absorbed in the intestine; body excretes excess, limiting the risk of toxicity.	Directly injected into muscle or skin; higher doses can cause temporary side effects.
Effectiveness	Lower absorption rates, making very high doses necessary for those with malabsorption.	Bypasses the digestive system, ensuring effective absorption, making it suitable for severe deficiencies.
Side Effects	Mild gastrointestinal symptoms like nausea or diarrhea are possible. Acne and rashes are rare side effects associated with high intake.	Mild, temporary side effects like dizziness, headache, or swelling are common. More severe side effects like irregular heartbeat are rare.
Medical Supervision	Often available over-the-counter; professional guidance is still recommended.	Requires a prescription and administration by a healthcare provider.

Drug Interactions Affecting Vitamin B12

Several medications can interfere with B12 absorption or metabolism, and it's essential to discuss them with a doctor. These include Metformin, Gastric Acid Inhibitors, Aminosalicylic Acid, Chloramphenicol, and Colchicine.

Other Scenarios for Caution

It's important to properly diagnose the cause of any suspected B12 deficiency, as treating without understanding the root cause could worsen the condition or mask more serious issues. High folic acid intake can also mask the signs of a B12 deficiency. Any history of allergic reactions should be discussed with a physician before starting supplementation. Taking B12 when levels are normal is unlikely to boost energy and won't address the real cause of fatigue.

Conclusion

While Vitamin B12 is a crucial nutrient, there are specific scenarios where it should be avoided. The most serious contraindications include allergies to cobalt or B12 and Leber's disease. Medical conditions like polycythemia vera and severe kidney or liver disease necessitate caution. Potential drug interactions with medications such as metformin and gastric acid inhibitors are also important to consider. Always consult with a healthcare professional to determine if supplementation is right for you, especially with an underlying health condition or other medications.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Please consult a healthcare professional for personalized guidance regarding vitamin B12 supplementation.

For further information on dietary supplements and health, the National Institutes of Health (NIH) Office of Dietary Supplements is a valuable resource.

Frequently Asked Questions

Individuals with advanced kidney disease should avoid high doses of vitamin B12 unless their doctor explicitly recommends it, as it can interfere with how the body processes the vitamin.

Yes, metformin, a medication for diabetes, can interfere with the absorption of vitamin B12. Long-term use can lead to deficiency, so patients should discuss B12 levels with their doctor.

Those with a cobalt allergy should not take vitamin B12, as cobalt is a component of the vitamin (cobalamin). Ingesting it can trigger an allergic reaction, which may be severe.

Yes, while rare, allergic reactions to B12 injections are possible, with severe cases potentially leading to anaphylaxis. It is important to inform a healthcare provider of any known allergies before receiving an injection.

People with Leber's disease should avoid vitamin B12 because some forms can worsen this hereditary condition that affects the optic nerve, potentially causing or aggravating vision loss.

Yes, taking high doses of folic acid can mask the symptoms of an underlying B12 deficiency, potentially allowing neurological damage to progress unnoticed.

While B12 is water-soluble and toxicity is rare, very high doses, especially from injections, can cause side effects like acne, rashes, headaches, nausea, or anxiety.