Vitamin / Neural Tube Defect Prevention Pregnancy: Folic acid deficiency during pregnancy may lead to the appearance of foetal malformations, and imbalance in folate-requiring trophoblast cells may lead to placental detachment. Very high doses have caused foetal abnormalities in rats, but harmful effects in the human foetus, mother or the pregnancy have not been reported following ingestion of folic acid. Folic acid is excreted in breast milk; no adverse effects have been observed in breast-fed infants whose mothers were receiving folic acid.

Folic Acid (Pre-conception and Pregnancy)

Brand names: Preconceive, Lexpec (5mg)

Used in: Anaemia

Folic acid taken before conception and during early pregnancy as a B-group vitamin supplement, given specifically to reduce the risk of fetal neural tube defects such as spina bifida.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Prevention of recurrence of neural tube defects: 5 mg daily, starting before conception and continuing throughout the first trimester of pregnancy

Route: Oral (the tablets are for oral use)

Frequency: Once daily

Max: Up to 15 mg daily may be necessary for malabsorption states (stated for folate-deficient megaloblastic anaemia)

Source: Folic Acid 5 mg Tablets SPC. This SPC covers the 5 mg strength only — it does NOT cover the lower-dose (e.g. 400 microgram) routine pre-conception/first-trimester supplementation used in women without additional risk factors, which must be sourced separately. Other adult indications stated in the same section: folate-deficient megaloblastic anaemia 5 mg daily for 4 months (up to 15 mg daily may be necessary for malabsorption states); prophylaxis in chronic haemolytic states or in renal dialysis 5 mg every 1-7 days depending on diet and underlying disease; drug-induced folate deficiency 5 mg daily. Folic acid may worsen the symptoms of co-existing vitamin B12 deficiency and should never be used to treat anaemia without a full investigation of the cause.

Paediatric dose

Dose: 500 micrograms/kg

Route: oral

Frequency: daily

Max: Not stated in source

UNIT IS MICROGRAMS PER KG (500 micrograms/kg = 0.5 mg/kg). Source states verbatim: 'Paediatric population — Over 1 year: As adult dose. Up to 1 year: 500 micrograms/kg daily.' Children over 1 year therefore take the adult dose for the indication concerned, not a per-kg dose.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Paediatric weight-based calculator

Patient weight (kg)

Verify in a children's formulary

Contraindications

Hypersensitivity to the active substance or to any of the excipients
Patients with malignant disease, unless megaloblastic anaemia due to folic acid deficiency

Side effects

May worsen the symptoms of co-existing vitamin B12 deficiency
Allergic reactions (rare): erythema, rash, pruritus, urticaria, dyspnoea and anaphylactic reactions including shock
Abdominal distension
Flatulence
Anorexia and nausea

Interactions

Sulfasalazine may reduce the absorption of folic acid
Cholestyramine may interfere with folic acid absorption — patients on prolonged cholestyramine therapy should take folic acid 1 hour before or 4 to 6 hours after cholestyramine
Trimethoprim or sulfonamides, alone or in combination as co-trimoxazole, may reduce the effect of folic acid — this may be serious in patients with megaloblastic anaemia
Serum levels of anticonvulsants (phenytoin, phenobarbital, primidone) may be reduced by folate — monitor carefully and adjust the anticonvulsant dose as necessary
Fluorouracil toxicity may occur in patients taking folic acid — this combination should be avoided
Edible clay or antacids containing aluminium or magnesium may reduce folic acid absorption — take antacids at least two hours after folic acid; folic acid may also reduce intestinal absorption of zinc (of particular importance in pregnancy)
Antibiotics may interfere with the microbiological assay for serum and erythrocyte folic acid concentrations, causing falsely low results

Clinical monograph

How it works

Folic acid is converted to tetrahydrofolate, an essential cofactor for nucleotide synthesis and methylation reactions required for rapid cell division and closure of the neural tube in early embryonic development.

Prescribing in practice

Supplementation should ideally begin before conception and continue through the first trimester, as the neural tube closes early in pregnancy, before many women know they are pregnant.
A higher-strength preparation is indicated for women at increased risk, including those with a previous affected pregnancy, diabetes, a high body-mass index, sickle cell disease, or taking antiepileptic or other antifolate medicines.
Folic acid can mask the haematological signs of vitamin B12 deficiency while allowing neurological damage to progress, so consider B12 status where deficiency is plausible.

Monitoring

Routine biochemical monitoring is not required for standard antenatal supplementation, the focus being on timely initiation and adherence.

Counselling the patient

Start taking folic acid as soon as you plan a pregnancy, not when you first miss a period.
Continue daily through the early weeks of pregnancy even if you feel well.
Tell your clinician if you have epilepsy, diabetes or a previous pregnancy affected by spina bifida, as you may need a stronger dose.

Evidence & guidelines

The landmark MRC Vitamin Study and subsequent guidance established that periconceptional folic acid substantially reduces the risk and recurrence of neural tube defects.

Reference: NICE NG3 Antenatal Care; NICE NG133; PHE Folic Acid Guidance; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

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