Folic acid supplements are generally safe when taken at recommended doses. But excessive intake can cause problems — from masking vitamin B12 deficiency to potentially affecting cancer risk.
Folic acid is the synthetic form of vitamin B9. It’s found in supplements and fortified foods like bread, cereal, and flour. Natural folate, on the other hand, comes from foods like leafy greens, beans, citrus fruits, and avocados.
Your body needs vitamin B9 for DNA synthesis, cell division, and red blood cell formation. It’s especially critical during pregnancy to prevent neural tube defects.
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Available on the App StoreBut here’s the concern: since mandatory fortification programs began in many countries, overall folic acid intake has increased significantly. And while folate from food is well-tolerated, synthetic folic acid behaves differently in your body.1
How folic acid accumulates in your body
Your body handles natural folate and synthetic folic acid differently. About 85% of folic acid from supplements gets absorbed, compared to roughly 50% of folate from food.
Once absorbed, folic acid travels to your liver for conversion into active forms. But your liver can only process so much at once. Taking high doses causes unmetabolized folic acid (UMFA) to build up in your bloodstream.1
This matters because elevated UMFA levels have been linked to several health concerns in research studies.
Understanding the upper limit
The National Institutes of Health sets the tolerable upper intake level for folic acid at 1,000 mcg daily for adults. This limit applies specifically to synthetic folic acid from supplements and fortified foods — not natural folate from food.
For context:
- The recommended daily amount is 400 mcg for most adults
- Pregnant women need 600 mcg
- Breastfeeding women need 500 mcg
Most people don’t exceed the upper limit through diet alone. The risk comes mainly from high-dose supplements, which can contain 800-5,000 mcg per pill.
1. Masking vitamin B12 deficiency
This is the most established concern with excess folic acid.2
Vitamin B12 and folate work together to produce red blood cells. When either is deficient, you develop a type of anemia called megaloblastic anemia — characterized by enlarged, abnormal red blood cells.
Here’s the problem: high folic acid intake can correct the blood cell abnormalities caused by B12 deficiency while leaving the underlying deficiency untreated. The anemia improves, so neither you nor your doctor suspects a problem.
Meanwhile, B12 deficiency continues damaging your nervous system. By the time neurological symptoms appear — numbness, tingling, balance problems, memory issues — permanent nerve damage may have occurred.
This risk is particularly relevant for:
- Older adults (B12 absorption decreases with age)
- Vegetarians and vegans (B12 comes primarily from animal foods)
- People taking certain medications that affect B12 absorption
- Those with digestive conditions like celiac disease
If you take folic acid supplements, periodically checking your B12 levels makes sense — especially if you’re over 50 or follow a plant-based diet.
2. Potential cognitive effects in older adults
Research suggests that high folic acid intake combined with low B12 status may accelerate cognitive decline in elderly adults.1
One study found that older people with high plasma folate levels and low B12 had worse anemia and cognitive impairment than those with normal B12 levels. The combination seemed to amplify B12 deficiency symptoms rather than simply mask them.
Another study reported that people with high folate and low B12 were up to 3.5 times more likely to experience cognitive decline compared to those with normal levels of both nutrients.
Suggested read: 9 Health Benefits of Vitamin B6 (Pyridoxine) Backed by Science
The mechanism isn’t fully understood, but researchers theorize that excess folic acid may interfere with B12’s neurological functions or that UMFA itself affects brain health.
This doesn’t mean older adults should avoid folic acid entirely. It means maintaining adequate B12 intake becomes even more important when taking folate supplements.
3. Concerns during pregnancy and child development
Folic acid supplementation during pregnancy prevents neural tube defects — that’s well-established and life-saving. Most prenatal vitamins contain 400-800 mcg, which falls within safe ranges.
However, some research raises questions about very high doses exceeding 1,000 mcg daily during pregnancy.3
One study found that children whose mothers took more than 1,000 mcg daily during pregnancy scored lower on cognitive development tests at ages 4-5 compared to children whose mothers took 400-999 mcg.
Another study linked higher maternal folate levels to increased insulin resistance in children ages 9-13.
These findings don’t suggest avoiding folic acid during pregnancy. Rather, they support sticking to recommended doses (400-800 mcg from supplements) unless your doctor advises otherwise for specific medical reasons.
Suggested read: Supplements During Pregnancy: What’s Safe and What to Avoid
4. The cancer connection: a complicated relationship
Folate’s relationship with cancer is nuanced. It appears to have opposite effects depending on timing and cancer status.4
Before cancer develops: Adequate folate protects cells by supporting proper DNA synthesis and repair. Low folate levels are associated with increased cancer risk.
After cancer begins: High folate intake may fuel existing cancer cell growth by providing the building blocks tumors need to multiply.
A large cohort study following over 1.4 million women found that those exposed to high-dose folic acid (more than 1 mg daily) had a 20% increased overall cancer risk. The risk for non-Hodgkin lymphoma was doubled.4
However, this doesn’t mean moderate folic acid supplementation causes cancer. Most studies find no increased risk at standard doses. The concern centers on:
- Very high doses (above the 1,000 mcg upper limit)
- People with a history of cancer or precancerous conditions
- Long-term high-dose supplementation
Eating folate-rich foods doesn’t carry these risks and may actually be protective.
5. Immune system effects
Emerging research suggests high folic acid doses may affect immune function. A clinical trial found that 5 mg daily (five times the upper limit) for 90 days reduced both the number and activity of natural killer cells — immune cells that help fight infections and cancer.5
More research is needed to understand whether lower doses have similar effects.
Who should be cautious
While most people don’t need to worry about folic acid toxicity from fortified foods and standard supplements, certain groups should pay closer attention:
- People over 60 — Higher risk of B12 deficiency and cognitive sensitivity to imbalanced B vitamins
- Those with low B12 levels — Folic acid can mask deficiency and worsen neurological symptoms
- Cancer survivors — May want to discuss folate intake with their oncologist
- Anyone taking high-dose supplements — Doses above 1,000 mcg require medical supervision
Safe supplementation guidelines
For most healthy adults:
- Stick to recommended doses — 400-800 mcg from supplements is typically sufficient
- Get folate from food first — Leafy greens, legumes, citrus fruits, and avocados are excellent sources with no upper limit concerns
- Monitor B12 status — Especially if you’re over 50, vegetarian, or taking high-dose folic acid
- Check your total intake — Account for fortified foods plus supplements
- Consult your doctor before exceeding 1,000 mcg daily
The bottom line
Folic acid is essential for health, and supplementation prevents serious birth defects. At recommended doses, side effects are rare.
The concerns arise with excessive intake — particularly when it masks B12 deficiency or when high doses are taken long-term. Staying within the 1,000 mcg daily limit and maintaining adequate B12 intake addresses most safety concerns.
When possible, prioritize folate from whole foods. Your body handles it differently than synthetic folic acid, and there’s no established upper limit for natural food sources.
Field MS, Stover PJ. Safety of folic acid. Ann N Y Acad Sci. 2018;1414(1):59-71. PubMed ↩︎ ↩︎ ↩︎
Mills JL, Von Kohorn I, Conley MR, et al. Low vitamin B-12 concentrations in patients without anemia: the effect of folic acid fortification of grain. Am J Clin Nutr. 2003;77(6):1474-1477. PubMed ↩︎
Valera-Gran D, García de la Hera M, Navarrete-Muñoz EM, et al. Folic acid supplements during pregnancy and child psychomotor development after the first year of life. JAMA Pediatr. 2014;168(11):e142611. PubMed ↩︎
Mortensen JH, Øyen N, Fomina T, et al. High-dose folic acid use and cancer risk in women who have given birth: A register-based cohort study. Am J Obstet Gynecol. 2024;232(3):327.e1-327.e12. PubMed ↩︎ ↩︎
Paniz C, Bertinato JF, Lucena MR, et al. A Daily Dose of 5 mg Folic Acid for 90 Days Is Associated with Increased Serum Unmetabolized Folic Acid and Reduced Natural Killer Cell Cytotoxicity in Healthy Brazilian Adults. J Nutr. 2017;147(9):1677-1685. PubMed ↩︎
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