Strong evidenceWomen's health

Folic Acid (Vitamin B9)

Periconceptional folic acid is one of the few supplements with an unambiguous, large, replicated benefit: it prevents neural tube defects. Beyond pregnancy, it lowers homocysteine but delivers little to no cardiovascular benefit in folate-fortified populations.

What the science says

Prevents neural tube defects Strong evidence
A Cochrane review of five trials in over 6,100 women found periconceptional folic acid reduced neural tube defects by about 70% (RR ~0.31), including a 68% reduction in recurrence for women with a previously affected pregnancy. The effect is large, consistent, and is the basis for mandatory grain fortification in over 80 countries.
Reduces stroke risk in folate-unfortified populations Moderate evidence
The CSPPT trial randomised 20,702 Chinese hypertensive adults in an unfortified setting to enalapril plus 0.8 mg folic acid versus enalapril alone and found a 21% reduction in first stroke over 4.5 years (2.7% vs 3.4%). Trials in fortified populations such as the US have generally been null, so the benefit appears confined to genuinely low-folate populations.
Prevents heart attacks or cardiovascular death Limited evidence
Folic acid reliably lowers homocysteine by roughly 25%, but large meta-analyses of homocysteine-lowering trials show no reduction in myocardial infarction or cardiovascular mortality. This is a textbook case of a surrogate marker moving while hard outcomes do not.
Improves mood or cognition in the general population Limited evidence
Trials of folic acid for depression prevention or cognitive decline in unselected adults have been largely negative. Any signal is confined to people with low folate status or those on antifolate drugs.

Dosage & safety

Studied doseRDA is 400 mcg DFE/day for adults. All women who could become pregnant should take 400-800 mcg/day of folic acid starting at least one month before conception and through the first trimester. Women with a previous NTD-affected pregnancy, or on antiepileptics, are usually advised 4-5 mg/day. CSPPT used 0.8 mg/day. Tolerable upper limit from supplements and fortified foods is 1,000 mcg/day of folic acid (the limit does not apply to food folate).

SafetyWell tolerated at standard doses. The key hazard is masking vitamin B12 deficiency: folic acid can correct the megaloblastic anaemia while the neurological damage of B12 deficiency progresses undetected — check B12 in older adults and anyone on metformin or a proton pump inhibitor before high-dose folate. Folic acid reduces the efficacy of methotrexate as a chemotherapy agent (though it is deliberately co-prescribed in rheumatology) and interacts with phenytoin, phenobarbital and pyrimethamine. Unmetabolised folic acid appears in serum at intakes above ~800 mcg/day; observational links to cancer promotion remain unproven but argue against megadosing. People with the MTHFR C677T variant still convert folic acid adequately in most cases — see L-methylfolate for the nuance.

Scientific references

Effects and safety of periconceptional oral folate supplementation for preventing birth defects
De-Regil LM et al. — Cochrane Database Syst Rev, 2015
Efficacy of folic acid therapy in primary prevention of stroke among adults with hypertension in China: the CSPPT randomized clinical trial
Huo Y et al. — JAMA, 2015

Where to buy

Vitamatic, Folic Acid, 400 mcg, 500 Tablets

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