Why Someone Would Avoid Folic Acid

October 9, 2025 •

4 min read

According to the Centers for Disease Control and Prevention (CDC), millions of people worldwide take folic acid supplements to prevent birth defects, but for some individuals, avoiding it is a necessary precaution for their health. Understanding why someone would avoid folic acid is crucial for managing specific health conditions, mitigating potential risks, and ensuring proper nutrient balance in the body.

Quick Summary

This article explains several critical reasons why certain individuals may need to avoid folic acid, including masking vitamin B12 deficiencies, interfering with medications, or potential risks associated with excessive intake.

Key Points

Masking B12 Deficiency: High doses of folic acid can mask megaloblastic anemia, a symptom of a co-existing vitamin B12 deficiency, potentially allowing irreversible neurological damage to progress unnoticed.
Cancer Concerns: Some research suggests that while moderate folate levels are protective, excessive folic acid intake may promote the growth of pre-existing cancer cells, though evidence remains mixed.
Drug Interactions: Folic acid can interfere with the effectiveness of medications like anticonvulsants (e.g., phenytoin) and chemotherapy drugs (e.g., methotrexate), necessitating avoidance or close monitoring.
Genetic Factors: Individuals with specific gene mutations, such as MTHFR polymorphisms, may have difficulty converting synthetic folic acid and may be better served by active folate forms like 5-MTHF.
Excessive Intake Side Effects: Taking high-dose folic acid supplements can cause adverse effects, including nausea, bloating, and in rare cases, allergic reactions.
Rare Malabsorption Conditions: People with hereditary folate malabsorption cannot absorb folate from food and require specialized medical treatment, making standard folic acid ineffective and potentially harmful.
Personalized Approach: A healthcare provider should be consulted to determine if folic acid is appropriate, especially for those with existing health conditions, genetic variants, or who take other medications.

Understanding the Differences: Folate vs. Folic Acid

Before diving into the reasons for avoidance, it's essential to distinguish between folate and folic acid. Folate is the naturally occurring form of vitamin B9 found in foods like leafy greens, beans, and oranges. Folic acid is the synthetic form used in supplements and fortified foods. While the body processes natural folate efficiently, synthetic folic acid requires conversion, and excessive intake can lead to unmetabolized folic acid (UMFA) accumulating in the blood. This distinction is critical to many of the potential health issues discussed below.

Masking Vitamin B12 Deficiency

One of the most significant and well-documented reasons to be cautious with folic acid is its ability to mask a vitamin B12 deficiency. Both folate and vitamin B12 deficiencies can cause megaloblastic anemia, a condition characterized by abnormally large red blood cells. High doses of folic acid can correct the anemic symptoms, causing the condition to appear resolved in blood tests. However, the neurological damage caused by a co-existing vitamin B12 deficiency would continue to progress undetected. This can lead to severe and irreversible nerve damage over time, including tingling, numbness, and cognitive impairment. For this reason, anyone with an undiagnosed anemia or at risk for B12 deficiency, such as older adults, vegans, or those with malabsorption issues, should be screened for both before supplementing with high-dose folic acid.

Potential Cancer Concerns

The relationship between folic acid and cancer is complex and a subject of ongoing research, leading some individuals, especially those with a history of cancer, to avoid high doses. Studies have shown that while adequate folate levels may protect against cancer initiation in healthy cells, high levels of folic acid might potentially promote the growth and spread of pre-existing, undiagnosed cancer cells. The timing of supplementation seems to be a crucial factor, as a high intake could fuel the growth of cancerous cells that are already present but have not yet been detected. This has led to concern following mandatory folic acid fortification programs in several countries. However, it's important to note that research on this topic is mixed, and high-quality studies are still needed to draw definitive conclusions.

Genetic Variations Affecting Metabolism

Some people possess genetic variations, most notably in the MTHFR (methylenetetrahydrofolate reductase) gene, which can impair their ability to convert folic acid into its active form, 5-methyltetrahydrofolate (5-MTHF). While the CDC states that common MTHFR variants are not a reason to avoid folic acid, some healthcare providers may recommend alternatives like 5-MTHF for better absorption in these individuals. For those with rare hereditary folate malabsorption, caused by a mutation in the SLC46A1 gene, the body cannot absorb folate from food at all, requiring specialized treatment rather than standard folic acid supplementation.

Interaction with Medications

Folic acid can interact with several types of prescription medications, potentially reducing their effectiveness or increasing their side effects. This is a critical reason why individuals on certain drug therapies are advised to avoid or carefully monitor their folic acid intake. A healthcare provider should always be consulted before starting a new supplement regimen, especially with existing prescriptions.

Folic Acid vs. Medication Interactions


Type of Medication	Folic Acid Interaction	Potential Consequence
Anticonvulsants (e.g., Phenytoin, Phenobarbital)	Can decrease blood levels of the medication.	Reduced efficacy of the seizure medication, potentially leading to increased seizure activity.
Chemotherapy Drugs (e.g., Methotrexate)	Folic acid and methotrexate have opposing functions, reducing the effectiveness of the chemotherapy.	Diminished cancer-fighting effects of the chemotherapy drug.
Antibiotics (e.g., Chloramphenicol)	Can interfere with the absorption and effectiveness of the antibiotic.	Decreased efficacy of the antibiotic treatment.
Antacids with Magnesium/Aluminum	Can stop the proper absorption of folic acid if taken within two hours.	Reduced effectiveness of folic acid supplementation.

Side Effects and Allergic Reactions

For some individuals, particularly at high doses, folic acid can cause unpleasant side effects such as nausea, diarrhea, and stomach upset. While rare, some people may experience a serious allergic reaction (anaphylaxis) to folic acid supplements, with symptoms including skin rashes, itching, and difficulty breathing. Anyone experiencing such symptoms should seek immediate medical attention.

Conclusion: A Personalized Approach to Supplementation

The decision to avoid folic acid is not a one-size-fits-all matter and depends on an individual’s unique health profile, genetics, and medication usage. While mandatory food fortification has successfully reduced neural tube defects, it has also increased overall folate intake, sometimes to excessive levels in certain populations. This highlights the importance of working with a healthcare provider to assess whether folic acid is the appropriate form of vitamin B9 for your needs or if alternatives, such as natural food folate or L-5-MTHF supplements, are a safer option. This personalized approach can help maximize the health benefits of this essential vitamin while minimizing potential risks.

Authoritative Link

For more information on folate and folic acid recommendations, consult the Centers for Disease Control and Prevention's resources on the topic.

Frequently Asked Questions

Yes, taking high doses of folic acid can cause side effects such as nausea, diarrhea, stomach upset, and bloating in some people. In rare cases, it can also cause allergic reactions like skin rashes or difficulty breathing.

While the MTHFR gene variant can reduce the body's ability to convert folic acid to its active form, the CDC states it's not a reason to avoid it. However, some doctors may recommend supplementing with the active form (5-MTHF) for better absorption.

Yes, this is a major concern. High doses of folic acid can correct the megaloblastic anemia caused by a B12 deficiency, allowing severe and irreversible neurological damage to progress undetected.

The evidence is mixed and controversial. Some studies suggest high intake of synthetic folic acid might promote the growth of existing, undiagnosed cancer cells, while adequate folate levels are protective. This is not a universal finding, and more research is needed.

Yes, folic acid can interact with certain drugs, including anticonvulsants like phenytoin and chemotherapy agents like methotrexate, which can reduce their effectiveness. It's crucial to consult a doctor before mixing supplements and medications.

Folate is the natural form of vitamin B9 found in food. Folic acid is the synthetic form used in fortified foods and supplements. The body processes them differently, and high intake of synthetic folic acid can lead to a buildup of unmetabolized folic acid in the blood.

This is a rare genetic disorder where the body cannot absorb folate from food, and standard oral folic acid is ineffective. Specialized treatment is required to bypass the absorption issue.