What Happens If You Take Folic Acid for a Long Time? Understanding Potential Risks

October 8, 2025 •

4 min read

Folic acid fortification has significantly reduced the incidence of neural tube defects globally. However, concerns have been raised about what happens if you take folic acid for a long time, particularly in high doses, and its potential for unintended health consequences. This synthetic form of folate behaves differently in the body than its natural counterpart.

Quick Summary

Extended, high-dose folic acid intake can lead to unmetabolized folic acid accumulation and may mask a vitamin B12 deficiency, potentially causing irreversible neurological damage and other health issues.

Key Points

Masks Vitamin B12 Deficiency: High doses of folic acid can correct the anemia symptoms of a B12 deficiency, allowing severe neurological damage to progress undetected.
Leads to Unmetabolized Folic Acid (UMFA): The body can only process a limited amount of synthetic folic acid, leading to UMFA accumulation with high intake from supplements and fortified foods.
May Accelerate Cognitive Decline: In individuals with low vitamin B12, high folic acid levels have been linked to a faster rate of age-related mental decline.
Complex Role in Cancer: Research suggests a dual role where adequate levels protect, but excessive levels might promote the growth of existing cancer cells, though evidence is mixed.
Potential Risks in Pregnancy: While essential for preventing birth defects, excessive maternal intake may be associated with negative effects on child neurodevelopment and insulin resistance.
Upper Intake Limit Exists: Health authorities have set a tolerable upper intake level (UL) for synthetic folic acid for adults, primarily to prevent masking B12 deficiency.

Folic Acid vs. Natural Folate: A Crucial Distinction

To understand the potential long-term effects of folic acid, it's essential to recognize the difference between folic acid and folate. Folate is the naturally occurring form of vitamin B9 found in foods like leafy greens, legumes, and eggs. Folic acid, on the other hand, is the synthetic version used in dietary supplements and to fortify grain products such as bread, flour, pasta, and cereals. The body absorbs folic acid much more efficiently than food folate.

While natural folate is easily metabolized by the body, the human body can only process a certain amount of synthetic folic acid at a time. The liver and other tissues convert folic acid into a usable form, but when intake exceeds this capacity, unmetabolized folic acid (UMFA) can accumulate in the bloodstream. This buildup is associated with several health concerns, especially with prolonged high-dose supplementation.

The Major Risk: Masking a Vitamin B12 Deficiency

One of the most well-documented and serious risks of long-term, high-dose folic acid use is its ability to mask a vitamin B12 deficiency. Both deficiencies can cause megaloblastic anemia, a condition characterized by abnormally large red blood cells. Taking large amounts of folic acid can correct the anemia symptoms, making the underlying B12 deficiency go undetected.

Because the anemia is resolved, the patient or doctor may not recognize the problem until more serious and irreversible neurological damage from the B12 deficiency becomes apparent. Vitamin B12 is critical for maintaining the myelin sheath that protects nerve fibers. Without it, this protection deteriorates, leading to progressive nerve damage that can cause numbness, tingling, cognitive decline, and balance issues. This risk is particularly high for older adults, who are already more prone to B12 malabsorption.

Potential Impact on Cognitive Function

Research has explored the link between high folate levels and cognitive health, particularly in the elderly. Several studies suggest that excess folic acid intake, especially in individuals with low vitamin B12 levels, may accelerate age-related mental decline. A study in older adults linked high blood folate levels to a higher risk of cognitive impairment in those with low vitamin B12 status. This combination of nutrient imbalance appears to be more damaging than low B12 alone. However, some research indicates that moderate folic acid supplementation may support cognitive function in older adults with pre-existing mild cognitive impairment, highlighting the importance of proper dosage and context.

Mixed Evidence Regarding Cancer Risk

Folic acid's role in cancer is complex and seems to have a dual effect. Adequate levels of folate are important for DNA synthesis and repair, which can help protect healthy cells from becoming cancerous. Conversely, some research indicates that high levels of folic acid might help pre-existing or cancerous cells to grow and spread. Studies have produced mixed results regarding the link between long-term, high-dose supplementation and increased cancer risk, particularly for prostate and colorectal cancers. It is speculated that the risk depends on the specific type of cancer and individual health history. For this reason, individuals with a history of cancer are often advised to avoid high-dose folic acid unless medically recommended.

Effects on Immune Function and Children's Health

Prolonged exposure to high levels of folic acid has also been associated with other health effects. One study in postmenopausal women suggested that excess folic acid from supplements could impair natural killer (NK) cell cytotoxicity, a component of the immune system. Furthermore, some studies raise concerns about the effects of very high maternal folic acid intake during pregnancy. While essential for preventing birth defects, excessive doses may be linked to increased insulin resistance and negatively affect neurodevelopment in children. As with other risks, more research is needed to fully understand the implications.

Safe Supplementation and Recommended Intake

The Tolerable Upper Intake Level (UL) for folic acid from supplements and fortified foods for adults is set by health authorities as a precaution. This limit is primarily intended to prevent the masking of B12 deficiency. There is no evidence of adverse effects from natural folate obtained from food. For most individuals, a balanced diet that includes folate-rich foods is sufficient. Specific recommendations for folic acid intake are often provided for pregnant women, with higher amounts potentially advised for those at high risk and under medical supervision.

Here is a comparison of folic acid and natural folate:


Feature	Folic Acid (Synthetic)	Natural Folate (Food Folate)
Source	Supplements and fortified foods	Leafy greens, citrus fruits, beans, etc.
Absorption Rate	High, approximately 85%	Lower, about 50%
Metabolism	Requires conversion by the liver; can lead to UMFA buildup with high doses	Easily metabolized by the body
UL	Established for adults to prevent masking B12 deficiency	No established upper limit
Associated Risks	Can mask B12 deficiency, potential links to cognitive and cancer issues with excess	No known health risks from excess intake through food

Conclusion

While folic acid is a crucial nutrient, particularly for preventing birth defects, and safe at recommended levels, long-term, high-dose supplementation is not without risk. The potential to mask a dangerous vitamin B12 deficiency is the most significant concern, especially for vulnerable populations like the elderly. Other potential issues include the accumulation of unmetabolized folic acid, which may affect immune and cognitive function. The distinction between synthetic folic acid and natural food folate is vital, as the risks are tied to the synthetic version. Therefore, it is always recommended to consult a healthcare provider before beginning long-term or high-dose supplementation to ensure it is appropriate for your individual health status and to monitor for any adverse effects.

CDC on Folic Acid Intake and Sources

Frequently Asked Questions

It is extremely difficult, if not impossible, to get too much natural folate from food sources alone. There is no established upper limit for folate from food, as the risks associated with high intake are specific to synthetic folic acid from supplements and fortified foods.

Older adults (over 50), individuals on vegan or vegetarian diets, people with certain gastrointestinal disorders, and those taking certain medications (like metformin) are at a higher risk of B12 deficiency and thus more susceptible to the masking effect of high folic acid intake.

UMFA is synthetic folic acid that has not been converted into a usable form by the body because intake exceeded the liver's processing capacity. It can build up in the bloodstream with long-term, high-dose supplementation.

Health organizations provide guidelines for the recommended daily intake of folate for most adults. Women who could become pregnant are often specifically advised on folic acid intake.

High folic acid can mask the megaloblastic anemia that causes symptoms like fatigue and weakness. It does not hide neurological symptoms, which may include numbness, tingling, balance problems, memory issues, or mood changes.

If you are in a high-risk group or taking folic acid supplements, it is crucial to also ensure adequate vitamin B12 intake. A doctor can test your B12 and folate levels to monitor your status. It is important to adhere to recommended intake levels for synthetic folic acid unless medically directed.

Getting folate from food is generally considered safer, as there is no risk of toxicity. For those with a higher need (e.g., pregnant women), supplements are effective. The safest approach is a balanced diet supplemented as needed and directed by a healthcare professional.