When Should You Not Take B12 Vitamins?

January 7, 2026 •

3 min read

According to research, while vitamin B12 is typically safe, some individuals should exercise caution or avoid supplementation altogether. Understanding when you should not take B12 vitamins is crucial for preventing negative health outcomes and adverse side effects, especially if you have pre-existing medical conditions or are taking certain medications.

Quick Summary

B12 supplementation may be contraindicated due to allergies, specific medical conditions, or drug interactions. Individuals with Leber's disease or certain blood disorders should avoid it, and caution is needed for those with kidney disease or specific medication use. Always consult a healthcare provider before starting B12.

Key Points

Allergy to B12 or Cobalt: Individuals with a known allergy or hypersensitivity to B12 or cobalt should never take supplements due to the risk of a severe allergic reaction.
Leber's Disease: Those with this hereditary eye condition, Leber's optic atrophy, should avoid B12, particularly cyanocobalamin, as it can worsen the disease.
Pre-existing Conditions: Patients with polycythemia vera, certain types of leukemia, or kidney disease should consult a doctor before supplementing, as B12 can interfere with these conditions.
High B12 Levels: If blood tests show naturally high B12 levels, do not supplement further. These elevated levels can be a sign of a serious, unaddressed health issue like liver disease.
Specific Drug Interactions: Be aware that certain medications, including gastric acid inhibitors (PPIs) and metformin, can affect B12 absorption or interact negatively with the supplement.
No Diagnosed Deficiency: If you eat a balanced diet and have no diagnosed B12 deficiency, supplementation is likely unnecessary. Since excess is excreted, megadoses are often a waste of money and can cause mild side effects.

When High B12 Levels Indicate Underlying Health Issues

Although vitamin B12 is a water-soluble vitamin and is generally considered safe, with the body excreting any excess, naturally high levels can sometimes signal a more serious underlying health issue. An abnormally high vitamin B12 status, beyond what is caused by simple supplementation, could be an early warning sign for conditions such as liver or kidney disease, or even certain types of leukemia. In these cases, taking more B12 is not the solution and could, in fact, mask or complicate the diagnosis of the primary condition. Monitoring B12 levels is crucial, and if they are consistently high without excessive supplementation, a full medical evaluation is warranted to treat the root cause rather than the symptom.

Medical Conditions That Contradict B12 Supplementation

Certain medical conditions represent clear contraindications for taking B12 vitamins. Foremost among these is Leber's disease, or hereditary optic nerve atrophy. Cyanocobalamin, a common form of B12, can exacerbate this condition and should be avoided. Patients with polycythemia vera, a blood disorder characterized by an overproduction of red blood cells, should also avoid B12 supplements as they can worsen the condition. Finally, those with significant kidney disease should be cautious, as their bodies may not properly excrete excess vitamins, and high doses could potentially cause issues.

Potential Drug Interactions with B12

Several medications can interfere with B12 absorption or function, making supplementation complex. For those on long-term medication, discussing B12 intake with a healthcare provider is essential.

Gastric Acid Inhibitors: Proton pump inhibitors (PPIs) like omeprazole (Prilosec) and lansoprazole (Prevacid), and H2-receptor antagonists like cimetidine (Tagamet), can decrease stomach acid. Since stomach acid is needed to release B12 from food, chronic use can lead to deficiency. While this often necessitates supplementation, it highlights a medication-related absorption issue.
Metformin: The common diabetes medication metformin has been shown to reduce B12 absorption in the long term, sometimes requiring supplementation to prevent deficiency.
Other Medications: Other drugs, including certain antiseizure medications (phenytoin, phenobarbital) and some antibiotics (chloramphenicol), can also lower B12 levels or interfere with its function.

Severe Side Effects and Allergic Reactions

Though rare, some individuals have severe allergic reactions to B12, particularly the injected form, which may contain cobalt. Anaphylaxis, a life-threatening allergic reaction, is a risk for those with sensitivity to B12 or cobalt. Symptoms can include hives, swelling, difficulty breathing, and a rapid heartbeat, and require immediate medical attention. Milder, but still unpleasant, side effects from very high doses can include headaches, skin issues like acne, and gastrointestinal distress.

Comparison of B12 Use Scenarios


Scenario	Is Supplementation Recommended?	Considerations and Risks
Leber's Disease	No	Can worsen the hereditary optic nerve atrophy.
Normal Health (No Deficiency)	Generally unnecessary	Excess B12 is excreted, but megadoses are costly and could cause side effects like acne.
Taking Metformin	Often needed, but requires supervision	Medication can impair absorption, so a doctor should monitor and prescribe the correct dose.
Known B12/Cobalt Allergy	No	Risk of severe allergic reaction, including anaphylaxis.
High B12 Levels (Endogenous)	No, but requires investigation	High levels can indicate liver disease, leukemia, or other serious health issues.

Conclusion: Prioritize Medical Consultation

While oral B12 supplements are a low-risk option for many, particularly those with a diagnosed deficiency or dietary restrictions, they are not without potential downsides. The body's ability to excrete excess B12 prevents typical overdose, but high levels can still cause issues or point to serious underlying diseases. Anyone with pre-existing conditions, especially Leber's disease or polycythemia vera, should avoid B12. Moreover, certain medications or a history of allergies warrant cautious use. The most prudent course of action is to consult a healthcare professional before beginning any new supplement regimen to ensure safety and effectiveness. This is particularly important for individuals considering injections or megadoses, as these carry a higher risk of adverse effects. Remember, supplementation should be based on need, not assumed benefit.

Mayo Clinic

Frequently Asked Questions

It is virtually impossible to consume dangerous or harmful levels of vitamin B12 from food sources alone, as the body's absorption capacity from food is limited.

Mild side effects from high-dose B12 supplements or injections can include headaches, skin issues like acne, nausea, and diarrhea.

For most healthy people without a deficiency, taking B12 supplements is unnecessary, as the excess is simply excreted. It is generally not harmful, but it can be a waste of money and may cause mild side effects with very high doses.

Gastric acid inhibitors, like omeprazole (Prilosec), reduce stomach acid needed to release B12 from food, which can impair absorption and lead to a deficiency over time.

B12 injections should be avoided by anyone with a hypersensitivity to vitamin B12 or cobalt, Leber's disease, or polycythemia vera.

Yes, abnormally high B12 blood levels can sometimes indicate an underlying condition, such as liver disease, kidney disease, or certain types of leukemia, and should be evaluated by a doctor.

B12 supplements can interact with several medications, including metformin and certain antiseizure drugs. Always inform your doctor about any supplements you are taking.