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Folic Acid (Pteroylmonoglutamic Acid)
The synthetic, fully oxidised form of vitamin B9 — the compound the neural-tube-defect prevention trials and food fortification actually used. It is NOT methylfolate: it needs dihydrofolate reductase to become active.
What the evidence says
Most Folic Acid studies are mechanism or observational rather than RCTs that measure a clinical effect — keep findings provisional.
Most evidence is from high-quality meta-analyses and randomised trials published 1991–2015.
Based on 4 studies · 1 meta-analysis · 3 RCTs
Confidence
Moderate confidenceBy outcome
For ONE indication — periconceptional prevention of neural tube defects — the evidence is about as strong as nutrition evidence gets: a randomised trial, a Cochrane synthesis, a US Preventive Services Task Force recommendation and mandatory national food fortification. The score is held at 8 rather than higher because this entry's other outcomes are not evidenced here.
Our research database did not respond in time, so the studies and count above cover only our curated set. This refreshes automatically.
Folic acid (pteroylmonoglutamic acid, C19H19N7O6, PubChem CID 135398658) is the synthetic form of vitamin B9. It is fully oxidised and biologically inert as supplied: dihydrofolate reductase must reduce it to tetrahydrofolate before the body can use it (StatPearls, NCBI Bookshelf NBK554487).
That is the difference from L-methylfolate (5-MTHF), which is already the circulating, reduced form — the two are listed separately here because they are different compounds with different evidence bases, and this entry carries the one that public-health policy and the prevention trials are built on.
The Institute of Medicine sets the adult RDA at 400 mcg dietary folate equivalents per day, 600 mcg DFE in pregnancy and 500 mcg DFE in lactation, and advises that anyone capable of becoming pregnant take 400 mcg of folic acid daily from fortified food, supplements, or both, in addition to food folate (NCBI Bookshelf NBK114318).
The United States has required enriched cereal-grain products to be fortified with folic acid at 140 mcg per 100 g since 1 January 1998 (FDA final rule, 61 FR 8781, 5 March 1996).
Outside the periconceptional indication, this entry makes no efficacy claim: the cardiovascular, stroke and cognitive literature has not been transcribed here.
Folic acid is supplied fully oxidised and is not directly usable. Dihydrofolate reductase converts it to tetrahydrofolate, the active form (StatPearls, NCBI Bookshelf NBK554487). This step is what methotrexate blocks.
Tetrahydrofolate carries one-carbon units for purine and thymidylate synthesis, which is why folate status limits rapidly dividing tissue — including the closing neural tube and the erythron.
Adequate folate around the time of conception lowers the risk of neural tube defects; the neural tube closes in the first weeks of pregnancy, which is why supplementation has to start before conception (USPSTF evidence review, NCBI Bookshelf NBK593617).
Actively recommended in pregnancy, rather than merely permitted. The IOM RDA in pregnancy is 600 mcg DFE/day (NCBI Bookshelf NBK114318), and the USPSTF advises 400-800 mcg of folic acid daily for anyone planning or capable of pregnancy, starting at least 1 month before conception (NCBI Bookshelf NBK593617).
The IOM RDA in lactation is 500 mcg DFE/day (NCBI Bookshelf NBK114318) and StatPearls states folic acid may be used as a supplement during breastfeeding (NBK554487).
The IOM sets lower upper limits for synthetic folic acid by age: 300 mcg/day at 1-3 years, 400 mcg/day at 4-8, 600 mcg/day at 9-13 and 800 mcg/day at 14-18 (NCBI Bookshelf NBK114318). A 400 mcg adult tablet already exceeds the limit for a child under 9.
No age-specific upper limit is set — the adult limit of 1,000 mcg/day applies (NCBI Bookshelf NBK114318). The reason to be careful is not the number: vitamin B12 deficiency is what the limit protects against, and folic acid can correct its anaemia while the neurological damage progresses (StatPearls NBK554487), so establish B12 status first.
Both interact with folate metabolism and both are prescribed alongside deliberate folate decisions. Take the dose your prescriber sets rather than adding a supplement independently.
Methotrexate works by inhibiting dihydrofolate reductase, the enzyme that converts folic acid to its active form (StatPearls, NCBI Bookshelf NBK556114). Folate supplementation alongside methotrexate is routinely prescribed for exactly this reason, but the dose and schedule are the prescriber's decision, not a self-selected one.
Deficient folate levels have been detected in up to 16% of patients taking antiepileptic drugs, including gabapentin, phenytoin, carbamazepine, valproate and primidone (StatPearls, NCBI Bookshelf NBK554487). Folate status is worth checking with the prescriber rather than self-corrected, because the interaction runs in both directions.
Folic Acid has an evidence score of 8/10 — strong evidence based on 4 indexed studies, including 1 meta-analysis. The synthetic, fully oxidised form of vitamin B9 — the compound the neural-tube-defect prevention trials and food fortification actually used. It is NOT methylfolate: it needs dihydrofolate reductase to become active. Representative study: PMID 26662928.
The commonly studied dose of Folic Acid is 400 mcg/day for anyone planning or capable of pregnancy (USPSTF: 400-800 mcg); adult RDA 400 mcg DFE/day, 600 mcg DFE in pregnancy, 500 mcg DFE in lactation (IOM). Individual needs vary — start at the lower end of the range and adjust based on how you respond.
Timing is flexible for Folic Acid — consistent daily use matters more than the time of day. Absorption is somewhat higher without food: the IOM's dietary folate equivalent factors are 0.5 mcg of a supplement taken on an empty stomach versus 0.6 mcg taken with a meal per 1 mcg DFE (NCBI Bookshelf NBK114318).
Folic Acid is generally safe at recommended doses, with a few precautions worth noting. Use caution if any of these apply to you: Untreated or undiagnosed vitamin B12 deficiency — rule B12 deficiency out before treating an apparent folate-deficiency anaemia with folic acid (StatPearls, NCBI Bookshelf NBK554487).
Vitamin C
Probably helpsHumans cannot synthesize this vitamin — required for collagen production, immune cell function, and iron absorption; athletes need more.
Thiamine
Probably helpsConverts carbohydrates into cellular energy and supports acetylcholine synthesis for healthy nerve and brain function.
Vitamin B12
Probably helpsDrives nerve myelination, red blood cell formation, and DNA synthesis — deficiency is common in vegans, vegetarians, and older adults.
Vitamin D3
Probably helpsHormone-like vitamin that regulates 1,000+ genes — deficiency is widespread and linked to poor immunity, depression, and bone loss.
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Reviewed by Dr. Baher Al Hakim · Last reviewed September 2026 · evidence from 4 studies · how we score · editorial policy
This information is for educational purposes only. It is not a substitute for professional medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any supplement or medication.