With methotrexate used for cancer
Safety flag
Supplemental folate can interfere with antifolate cancer therapy.
Interaction evidence source All research profiles
Vitamin
Folic acid
Prepared by the ezGLP team. Sources last checked . Profile reviewed .
Also known as Vitamin B9, Folate
Routes represented in the knowledgebase: Oral / Subcutaneous (SubQ).
Educational overview
Supplement and prescription replacement; high intake can mask anemia while neurologic injury from B12 deficiency continues.
Folate is the umbrella name for this B vitamin in all its chemical forms; folic acid is the fully oxidised synthetic version, pteroylmonoglutamic acid, which occurs in supplements and fortified foods rather than in nature. Food folates are reduced polyglutamates, mainly 5-methyltetrahydrofolate, and are less stable and less completely absorbed, which is why intakes are expressed as dietary folate equivalents. Folic acid must be reduced by dihydrofolate reductase before entering one-carbon metabolism, where tetrahydrofolate derivatives donate carbon units for purine and thymidylate synthesis and for the remethylation of homocysteine to methionine. Rapidly dividing tissues, including bone marrow and the developing neural tube, are the most sensitive to shortfall.
The decisive trial is the Medical Research Council Vitamin Study, published in the Lancet in 1991, in which periconceptional folic acid cut recurrent neural tube defects by roughly seventy percent. The US Preventive Services Task Force reaffirmed in 2023 that periconceptional folic acid supplementation prevents neural tube defects, a grade A conclusion, and population data after mandatory fortification show a marked fall in affected pregnancies. Evidence outside that indication is weaker. Large homocysteine-lowering trials combining folic acid with vitamins B6 and B12 lowered homocysteine but did not reduce cardiovascular events, and analyses of folic acid intake and cancer risk have produced inconsistent findings that remain debated.
Regulatory treatment is unusually layered. FDA amended the standards of identity for enriched grain products in 1996, so flour, bread, pasta, rice, and cereal labelled enriched have carried added folic acid since 1998; in 2016 the agency separately authorised voluntary folic acid fortification of corn masa flour as a food additive. Oral folic acid is sold as a dietary supplement at nutrient levels and as a prescription tablet at higher strengths. Folic Acid Injection USP is an FDA-approved product marketed under abbreviated new drug applications for intramuscular, intravenous, or subcutaneous administration in megaloblastic anemia, which distinguishes it from the compounded vitamin blends sold by wellness clinics. The European Union has no mandatory fortification programme.
Folic acid degrades with light, heat, and humidity, so tablets are kept in the closed original container and the injection is labelled for controlled room temperature with protection from light. The tolerable upper intake level published by NIH applies to synthetic folic acid from supplements and fortified foods rather than to folate from unfortified food, so an inventory record listing every folic-acid-containing product, including prenatal vitamins, B-complex products, and fortified cereals, is the practical way to see a total. Two entries carry clinical weight: concurrent methotrexate, because supplemental folate interacts with antifolate therapy, and vitamin B12 status, since ample folic acid can correct the anemia of B12 deficiency while neurological damage continues.
- Category
- Vitamin
- Routes represented
- Oral / Subcutaneous (SubQ)
- Storage context
- Follow the product or pharmacy label; requirements vary by route and formulation.
Published literature
No verified PubMed or DOI link is available for this entry.
Safety evidence
Serious or important: masking of B12-related anemia while neurologic injury continues.
Safety evidence sourceInteraction evidence
These are general, pairwise evidence notes. Green identifies the potential-synergy category, while Lv3/Lv2/Lv1 communicates evidence strength; it does not guarantee benefit or cancel a safety warning.
With cyanocobalamin (vitamin B12)
Complementary evidenceLv3
A randomized human trial found folic acid plus vitamin B12 lowered homocysteine more than folic acid alone.
Interaction evidence sourceCardiovascular outcome trialCardiovascular outcome trialCommon questions
What is the difference between folic acid and folate?
Folate is the general name for the vitamin in all its chemical forms, including the reduced polyglutamates found naturally in leafy greens, legumes, and liver. Folic acid is the fully oxidised synthetic form found in supplements and fortified grain products; it is more stable and more completely absorbed, so intake figures are converted into dietary folate equivalents to allow comparison. Both feed the same one-carbon metabolism after reduction inside cells.
Can folic acid mask a vitamin B12 deficiency?
Yes, and this is a principal reason a tolerable upper intake level exists for synthetic folic acid. Both vitamins are required for red cell maturation, so ample folate can resolve the megaloblastic anemia caused by B12 deficiency while the separate neurological injury of that deficiency progresses undetected. The NIH Office of Dietary Supplements describes this masking effect, and a 2024 review in Food and Nutrition Bulletin re-examines its significance in fortified populations.
Does an MTHFR gene variant change how folic acid works?
The common C677T variant of methylenetetrahydrofolate reductase lowers enzyme activity, and people carrying two copies tend to show lower red cell folate and higher homocysteine, particularly when folate intake is low. Folic acid still raises folate status in carriers. The American College of Medical Genetics and Genomics published a practice guideline in 2013 concluding that evidence does not support MTHFR polymorphism testing in clinical practice, including for thrombophilia or recurrent pregnancy loss.
Is there an FDA-approved folic acid injection?
Yes. Folic Acid Injection USP is marketed under abbreviated new drug applications for intramuscular, intravenous, or subcutaneous administration, indicated for megaloblastic anemias arising from folate deficiency when the oral route is not suitable. That is a different regulatory category from the compounded multi-vitamin injections offered by wellness clinics, which carry no FDA approval and are prepared under state pharmacy rules.
Why is folic acid added to bread and cereal?
To prevent neural tube defects in pregnancies before they are recognised, since the neural tube closes within the first weeks. FDA amended the standards of identity for enriched grain products in 1996, with compliance required by 1998, so enriched flour, bread, pasta, rice, and cereal in the United States contain added folic acid. Corn masa flour was authorised for voluntary fortification in 2016.
Further reading
- NIH Office of Dietary Supplements, Folate fact sheet for health professionals
- MRC Vitamin Study Research Group, prevention of neural tube defects, Lancet 1991
- US Preventive Services Task Force, folic acid to prevent neural tube defects, JAMA 2023
- Hickey et al., ACMG practice guideline on MTHFR polymorphism testing, Genet Med 2013
- Miller & Green, excess folic acid and vitamin B12 deficiency, Food Nutr Bull 2024
- FDA, Folate and Folic Acid on the Nutrition and Supplement Facts Labels
- FDA, Fortifying Corn Masa Flour Products with Folic Acid
- DailyMed, Folic Acid Injection USP (Fresenius Kabi) label
Summarised from published literature for reference only, with citations to the source. Evidence is graded as reported and is not a recommendation to combine, dose, or use any compound.
For personal tracking and education only. Not medical advice, diagnosis, or treatment, and not a medical device. Peptides may be prescription-only or unapproved where you live. Consult a licensed healthcare professional.