Who should avoid taking folic acid and why it matters

November 14, 2025 •

4 min read

While folic acid is widely known for its role in preventing birth defects and supporting cell growth, certain medical conditions and circumstances make supplementation risky. It is critical to understand who should avoid taking folic acid to prevent serious health complications, such as masking a dangerous vitamin B12 deficiency.

Quick Summary

People with specific health issues, including vitamin B12 deficiency, certain cancers, and epilepsy, should use extreme caution with folic acid supplements and consult a doctor. High doses can pose significant risks. Always seek professional medical advice before starting any new supplement regimen to ensure it is safe for your unique health profile.

Key Points

Vitamin B12 Deficiency Masking: Unsupervised folic acid can hide the neurological damage of an untreated vitamin B12 deficiency by normalizing blood tests, leading to severe and irreversible nerve damage.
Cancer Concerns: Individuals with a history of cancer should avoid high-dose folic acid, as some studies suggest it may accelerate tumor growth or interfere with chemotherapy treatments.
Epilepsy and Seizure Risk: High doses of folic acid can potentially worsen seizures and interfere with certain anti-epileptic medications, posing a risk for individuals with seizure disorders.
Drug Interactions: Folic acid can interact with a variety of medications, including those for cancer, epilepsy, and inflammatory bowel conditions, altering their effectiveness.
Genetic Considerations: People with an MTHFR gene mutation, while not necessarily needing to avoid folic acid, should consult a doctor to determine if an activated folate form (L-5-MTHF) might be more appropriate.
Pre-Angioplasty and Dialysis: Specific health situations, such as recovering from an angioplasty or being on certain types of kidney dialysis, require medical consultation before taking folic acid.

Folic acid, the synthetic form of vitamin B9, is a crucial nutrient, especially during pregnancy, to prevent serious birth defects. However, it is not a one-size-fits-all supplement. For some individuals, taking folic acid can lead to significant health problems, not all of which are immediately obvious. The decision to supplement should always be made in consultation with a healthcare provider, particularly for those with pre-existing conditions or those on specific medications. Understanding the risks is the first step toward safeguarding your health.

The Critical Link with Vitamin B12

One of the most dangerous risks associated with inappropriate folic acid supplementation is its ability to mask the symptoms of a vitamin B12 deficiency. Both vitamin B12 and folate are essential for the production of healthy red blood cells. When a person is deficient in vitamin B12, it can lead to a type of anemia where the red blood cells are abnormally large. This is often accompanied by neurological symptoms, which can include memory loss, confusion, and nerve damage.

The Masking Effect Explained

When a person with an underlying vitamin B12 deficiency takes folic acid, the folic acid can correct the anemic symptoms (the blood disorder) without fixing the vitamin B12 problem. This creates a 'false sense of security,' as the person may feel better and their blood work may appear normal, while the irreversible neurological damage from the untreated vitamin B12 deficiency silently continues to progress. This is why a healthcare provider will often test for vitamin B12 levels before recommending folic acid therapy, especially for those with unexplained anemia. People with pernicious anemia, an autoimmune condition that prevents the absorption of vitamin B12, are particularly at risk.

Pre-existing Conditions that Demand Caution

Certain medical conditions can make folic acid supplementation inadvisable or require specific medical supervision.

Cancer

Research on the relationship between folic acid and cancer is complex and has raised concerns, particularly regarding high-dose supplementation. While folate deficiency has been linked to cancer development, some studies suggest that high levels of folic acid may accelerate the growth of existing tumors. For this reason, individuals with a history of cancer should avoid high doses of folic acid unless specifically advised by their oncologist. Concerns exist particularly for colorectal and breast cancer survivors.

Seizure Disorders

Patients with epilepsy or other seizure disorders should be cautious with folic acid supplements. High doses may worsen seizure activity and interfere with the effectiveness of certain anti-epileptic medications. Any changes to medication or supplementation should be made with a doctor's guidance.

Kidney Disease

Individuals undergoing certain types of kidney dialysis, such as haemodialysis, should not take folic acid without a doctor's approval. Additionally, those recovering from angioplasty, a procedure to widen narrowed arteries, are advised against taking folic acid along with other B vitamins.

Genetic Mutations (MTHFR)

Some people have a common genetic mutation in the methylenetetrahydrofolate reductase (MTHFR) gene, which can affect how their body processes folic acid. While this is a complex issue, the CDC clarifies that people with common MTHFR variants can still process folic acid and benefit from supplementation, particularly in preventing neural tube defects. However, those with this mutation may benefit from an activated form of folate, L-5-MTHF, and should consult their healthcare provider for personalized advice.

Comparison of Folic Acid Use by Condition


Condition / Risk Factor	Folic Acid Recommendation	Rationale and Precaution
Trying to Conceive / Early Pregnancy	Highly Recommended: 400-800 mcg daily	Prevents neural tube defects in the fetus; often combined with prenatal vitamins.
Vitamin B12 Deficiency / Pernicious Anemia	AVOID without proper B12 treatment	Masks neurological damage, allowing severe and irreversible nerve damage to progress undetected.
History of Cancer	AVOID High Doses; Consult Oncologist	High intake may potentially fuel cancer growth or interfere with chemotherapy treatments.
Epilepsy / Seizure Disorder	AVOID High Doses; Consult Neurologist	May worsen seizures and interact with anticonvulsant medications.
MTHFR Genetic Mutation	Seek Doctor's Advice; Consider L-5-MTHF	While folic acid can still be processed, an active form of folate (L-5-MTHF) may be more beneficial for those with significant processing difficulties.

Medications with Potential Interactions

Folic acid supplements can also interact with certain prescription medications, potentially altering their effectiveness or causing adverse effects. You should always inform your doctor of any supplements you are taking. Medications known to interact include:

Certain anticonvulsants (e.g., phenytoin, phenobarbital, primidone)
Chemotherapy drugs (e.g., methotrexate, fluorouracil)
Sulfasalazine, for inflammatory bowel conditions
Some antibiotics (e.g., tetracycline)
Cholestyramine, for high cholesterol

Conclusion: Consult Your Doctor

While folic acid is a vital nutrient for many, it is not without risks for specific populations. The most serious concern is its ability to conceal a dangerous vitamin B12 deficiency, potentially leading to irreversible neurological damage. Furthermore, individuals with a history of cancer, seizure disorders, specific genetic variations, or those taking certain medications must proceed with extreme caution. The safest and most responsible approach is to always discuss your individual health needs and supplement plans with a qualified healthcare professional. For additional guidance, consider resources like the NHS's information on who can and cannot take folic acid.

Disclaimer: This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.

Frequently Asked Questions

No, you should not take folic acid alone if you have an unaddressed vitamin B12 deficiency. It can mask the blood-related symptoms of the deficiency while allowing neurological damage to progress irreversibly. Always have a healthcare provider check your B12 levels before starting folic acid supplements.

While the link is complex, some research suggests that high daily doses of folic acid (above 0.8-1.2 mg) might increase the risk of certain cancers or accelerate the growth of existing tumors. Individuals with a history of cancer should avoid high doses unless advised by their oncologist.

High doses of folic acid can increase the risk of seizures and may decrease the effectiveness of anti-epileptic drugs like phenytoin and primidone. Anyone with epilepsy should consult their neurologist before taking folic acid.

People with a history of cancer, epilepsy, certain heart conditions (like recovering from angioplasty), or a vitamin B12 deficiency should be especially cautious with high doses of folic acid.

Not necessarily. While the MTHFR mutation can affect folate metabolism, the CDC indicates that most individuals can still benefit from standard folic acid fortification and supplementation. However, some may benefit more from L-5-MTHF, an active form of folate, and should consult a doctor.

Yes, folic acid can interact with various medications, including certain chemotherapy drugs like methotrexate, anticonvulsants, and sulfasalazine. It is essential to inform your doctor of all supplements you are taking.

It is most important to consult a doctor if you have a diagnosed or suspected vitamin B12 deficiency, a history of cancer, a seizure disorder, kidney disease, or are on any long-term prescription medication.