What Happens If You Get Too Much Folate?

November 17, 2025 •

4 min read

While folate is a crucial nutrient, an excess of its synthetic form, folic acid, can pose health risks, including masking a serious vitamin B12 deficiency. High levels are most often caused by over-supplementation, not a diet rich in natural folate from foods.

Quick Summary

Excess folic acid can mask a B12 deficiency, potentially causing irreversible neurological damage. High intake is also linked to risks during pregnancy, accelerated cognitive decline, and some cancers.

Key Points

B12 Deficiency Masking: Excess folic acid can correct the anemia of a B12 deficiency but allows nerve damage to progress undetected.
Natural vs. Synthetic: Natural folate from food is safe in high amounts, but synthetic folic acid can accumulate in the body and become problematic at high doses.
Cognitive Risks: High folic acid levels are linked to accelerated mental decline, particularly in older adults with low B12 status.
Pregnancy Concerns: Excessive maternal folic acid intake is associated with potential risks to the child's neurodevelopment and metabolism, including an increased risk for autism and insulin resistance.
Monitoring is Key: Regular blood tests for both folate and vitamin B12 are recommended, especially for those who take supplements, to ensure balanced B-vitamin status.
Supplement vs. Diet: It is safest to meet folate needs primarily through a varied diet of whole foods rather than high-dose supplements.

What is the Difference Between Folate and Folic Acid?

To understand the risks of excess intake, it is vital to distinguish between natural folate and its synthetic counterpart, folic acid. Folate is the naturally occurring form of vitamin B9 found in various foods such as leafy greens, legumes, and citrus fruits. Folic acid is the man-made form added to supplements and fortified foods like bread, cereals, and pasta. The body processes these forms differently. The liver can only metabolize a certain amount of folic acid at a time, so excessive intake can lead to a buildup of unmetabolized folic acid (UMFA) in the bloodstream. This does not occur with natural folate from whole foods.

The Dangers of Excess Synthetic Folic Acid

Exceeding the Tolerable Upper Intake Level (UL) of 1,000 mcg per day for folic acid can lead to several health issues. These risks primarily relate to high intake from supplements and fortified foods, as natural folate from diet has no known upper limit.

Masking Vitamin B12 Deficiency: This is arguably the most significant risk. High folic acid intake can correct the megaloblastic anemia associated with vitamin B12 deficiency, which can cause symptoms like fatigue and weakness. However, it does not treat the underlying B12 deficiency, allowing it to progress unnoticed. If left untreated, a B12 deficiency can cause severe and irreversible nerve damage to the brain and nervous system. This is particularly concerning for older adults and those with absorption issues.
Accelerated Cognitive Decline: Studies suggest that excess folic acid intake can accelerate age-related mental decline, especially in older adults with low vitamin B12 levels. The combination of high folate and low B12 has been linked to a higher risk of losing brain function compared to those with normal levels.
Potential Link to Certain Cancers: Research on the relationship between high folic acid and cancer risk is mixed and complex. Some studies suggest that while sufficient folate may protect against cancer, high doses of synthetic folic acid may help pre-existing cancer cells grow or spread. This is thought to be more of a concern for individuals with a history of cancer, particularly prostate or colorectal cancer.
Side Effects and Accumulation of Unmetabolized Folic Acid (UMFA): The buildup of UMFA in the blood from high folic acid intake has been linked to potential immune system impacts and other health concerns, though more research is needed. Common side effects associated with high doses can include stomach issues, irritability, sleep problems, confusion, and skin reactions.

High Folic Acid Intake During Pregnancy

While adequate folate is crucial for preventing neural tube defects in infants, excessive intake during pregnancy has raised concerns. Studies have found potential associations between very high maternal folate levels and increased risks for the child later in life. Research by the Johns Hopkins Bloomberg School of Public Health, for instance, suggested that very high folate and B12 levels in mothers could multiply the risk of the child developing an autism spectrum disorder. Other studies have linked excessive folic acid supplementation to impaired neurocognitive development in children.

Comparison: Natural Folate vs. Synthetic Folic Acid


Feature	Natural Folate (from food)	Synthetic Folic Acid (from supplements/fortified food)
Source	Dark leafy greens, legumes, fruits, eggs	Fortified grains, cereals, supplements
Absorption	Less readily absorbed by the body (about 50%)	Very well absorbed (about 85%)
Processing	Converted more efficiently in the body	Liver has a limited capacity, can lead to UMFA buildup
Known Upper Limit	No established upper limit; no known risks from food	Upper limit of 1,000 mcg (1 mg) for supplements and fortified food
Safety Concerns	Minimal risk of toxicity from food sources alone	Potential risks from high intake, including UMFA accumulation and masking B12 deficiency

Managing and Preventing High Folate Levels

If a blood test reveals high folate levels, management focuses on reducing unnecessary intake. Because folate is water-soluble, excess amounts are typically excreted through urine, and levels often normalize within a few weeks of adjusting intake.

Review Intake: Check all supplements (multivitamins, B-complex) and fortified food labels for folic acid content. Many people unknowingly exceed the UL from multiple sources.
Adjust Supplements: Your healthcare provider may advise reducing the dose or temporarily stopping supplements. For those who require supplementation, such as pregnant women, using the active form, 5-methyltetrahydrofolate (5-MTHF), may be a better option as it bypasses potential metabolic issues and doesn't mask a B12 deficiency.
Prioritize Natural Sources: Focus on getting your folate from a diverse diet of natural, folate-rich foods to avoid the risks associated with synthetic intake.
Test B12 Levels: If high folate is detected, it is critical to also test vitamin B12 levels to check for a co-existing deficiency that may have been masked.

Conclusion

While adequate folate is essential for health, especially for pregnant women, it is crucial to recognize that more is not always better when it comes to synthetic folic acid. The potential to mask a serious vitamin B12 deficiency, accelerate cognitive decline, and affect unborn children underscores the need for careful management. By prioritizing natural food sources of folate and using supplements responsibly under a healthcare provider's guidance, individuals can achieve optimal folate status without the associated risks of excess synthetic intake.

For more in-depth medical information on folic acid, consult authoritative resources like the NIH Office of Dietary Supplements.

Frequently Asked Questions

While often without symptoms, high folate intake, particularly from supplements, can cause gastrointestinal issues like nausea and cramps, irritability, sleep disturbances, skin reactions, and confusion.

It is extremely rare to get too much folate from natural food sources alone, as the body processes it differently and excretes excess amounts more easily. The risks are primarily associated with high-dose synthetic folic acid from supplements and fortified foods.

High intake of synthetic folic acid can correct the blood changes (megaloblastic anemia) seen in B12 deficiency, so the tell-tale symptom disappears. This allows the neurological damage caused by the untreated B12 deficiency to continue to progress silently.

UMFA is the synthetic folic acid that the body has not been able to process into its active form. It can build up in the bloodstream when intake exceeds the liver's capacity, and its long-term effects are a subject of ongoing research.

Consult a healthcare provider to review all sources of folic acid in your diet and supplements. They may recommend reducing your intake or switching to a supplement with the active form, 5-MTHF, and checking your B12 levels.

Yes. While adequate intake is critical, studies have linked excessively high maternal folate levels to increased risks for the child, including effects on neurodevelopment, metabolism, and an increased risk of autism spectrum disorder.

For adults, the Tolerable Upper Intake Level (UL) for folic acid from supplements and fortified foods is set at 1,000 mcg (1 mg) per day. This limit does not apply to natural folate found in foods.