The Critical Interaction: Why should you not take B12 and folic acid together?

October 1, 2025 •

3 min read

According to the National Institutes of Health (NIH), excessive intake of folic acid can obscure or mask a vitamin B12 deficiency, delaying its diagnosis until irreversible neurological damage occurs. This masking effect is the single most important reason why you should not take B12 and folic acid together in high doses without a proper medical evaluation.

Quick Summary

High folic acid intake can correct the anemia associated with vitamin B12 deficiency, hiding the underlying problem and allowing neurological damage to worsen unnoticed.

Key Points

Masking Effect: High doses of folic acid can hide the megaloblastic anemia symptom of a B12 deficiency, delaying a critical diagnosis.
Neurological Damage: The underlying B12 deficiency continues to cause progressive and potentially irreversible nerve damage even if the anemia is corrected by folic acid.
At-Risk Groups: Older adults, vegans, and individuals with malabsorption issues are at higher risk for this dangerous interaction.
Pre-supplementation Testing: It is crucial to get both B12 and folate levels tested before starting supplementation to avoid masking a deficiency.
Proper Treatment: If a B12 deficiency is confirmed, it must be treated directly with B12 injections or high-dose oral supplements, not masked with folic acid.
Homocysteine and Cognitive Health: The negative interaction between high folate and low B12 can worsen cognitive impairment and biochemical markers associated with B12 deficiency.

The Intricate Roles of Vitamin B12 and Folate

Both vitamin B12 and folate (vitamin B9) are essential B-complex vitamins vital for DNA synthesis, red blood cell formation, and nervous system function. Their metabolic pathways are interconnected, especially in the methylation cycle. While often found together in supplements due to this relationship, this interaction can be risky in cases of undiagnosed B12 deficiency.

The 'Masking Effect' Explained

The main risk of taking high doses of folic acid with a B12 deficiency is the "masking effect". Both deficiencies can cause megaloblastic anemia, but only B12 deficiency leads to severe neurological damage. High folic acid can correct the anemia symptom in B12 deficient individuals, potentially misleading medical evaluation as the most visible sign disappears.

However, the hidden B12 deficiency continues to harm the nervous system, potentially causing permanent nerve damage and cognitive issues. This damage is not treated by folic acid, and the delay in diagnosing B12 deficiency can have serious long-term consequences. This is why treating pernicious anemia solely with high-dose folic acid was stopped decades ago.

Why Neurological Damage Worsens

The interaction between these vitamins can also worsen neurological symptoms. High levels of unmetabolized folic acid have been linked to poorer cognitive function in older adults with low B12. Studies also suggest that high folate with low B12 is linked to a more severe biochemical deficiency, potentially by affecting B12 transporters. This mechanism might explain the rapid neurological decline seen historically when high folic acid was given to B12-deficient patients.

Who is at Elevated Risk?

Certain individuals are at higher risk and should have their B12 levels checked before taking high-dose folic acid. These groups include:

Older Adults: More likely to have B12 malabsorption and may take multivitamins with folic acid.
Vegans and Vegetarians: Increased risk of B12 deficiency due to diet; need B12 supplementation and monitoring.
Individuals with Gastrointestinal Disorders: Conditions like Crohn's or celiac disease can impair B12 absorption.
Those on Certain Medications: Some drugs can interfere with B12 absorption.

Important Signs of B12 Deficiency to Monitor

Even if anemia is corrected, be aware of these potential signs of B12 deficiency:

Pins and needles, numbness, or tingling
Memory loss, confusion, or difficulty concentrating
Fatigue and lack of energy
Muscle weakness
Vision problems
Psychological issues like depression

Comparison of Deficiency Effects


Deficiency Type	Anemia Status	Neurological Symptoms	Primary Cause	Treatment Approach
Pure B12 Deficiency	Megaloblastic Anemia	Present and progressive	Malabsorption, dietary insufficiency	B12 injections or high-dose oral supplementation
Pure Folate Deficiency	Megaloblastic Anemia	Absent	Poor diet, malabsorption	Folic acid supplementation
Masked B12 Deficiency	Anemia corrected	Present and progressive	B12 deficiency concealed by high folic acid	Requires proper B12 treatment to prevent neurological damage

Safe Supplementation Practices

The safest approach is to test your vitamin levels before starting supplements. A blood test can measure both B12 and folate. For those needing high-dose folic acid, B12 is often included to prevent neurological issues. For most, a balanced diet and standard multivitamin are sufficient. Consult a healthcare professional if you have concerns about your vitamin levels.

Conclusion

While B12 and folic acid are crucial, taking them together requires caution due to the risk of folic acid masking a B12 deficiency and allowing irreversible neurological damage. Blood tests and medical assessment are key to safe supplementation, especially for vulnerable groups. Understanding this interaction is vital for informed health decisions. The NHS advises checking B12 levels before beginning folic acid to prevent this issue.

Frequently Asked Questions

High folic acid itself does not cause nerve damage, but it can mask a co-existing B12 deficiency by correcting the anemia it causes. This allows the nerve damage from the untreated B12 deficiency to progress undetected.

The primary danger is the masking effect, where high folic acid corrects the blood picture (megaloblastic anemia) but leaves the underlying B12 deficiency untreated, allowing irreversible neurological damage to continue.

Older adults, individuals on a vegan or vegetarian diet, and those with gastrointestinal conditions affecting nutrient absorption (like Crohn's or Celiac disease) should be most vigilant.

Untreated B12 deficiency can lead to a range of neurological issues, including permanent nerve damage, cognitive decline, memory loss, confusion, and pins and needles sensations.

A doctor can order a simple blood test to check your vitamin B12 and folate levels simultaneously. This is the safest way to determine your status before starting any supplementation regimen.

For most healthy people, standard doses in a B-complex are safe. The risk arises with high-dose supplementation in individuals with an unknown B12 malabsorption issue or dietary inadequacy. Always consult a healthcare provider.

In most cases, a healthcare professional will first prescribe a form of B12 therapy, such as injections, to address the deficiency. They may then prescribe a specific dose of folic acid if needed, ensuring the underlying B12 issue is controlled.