Folic acid

What is Folic acid?

Folate is vitamin B9. The word covers the forms found naturally in food — broccoli, Brussels sprouts, leafy greens, peas, chickpeas, kidney beans and liver — and folic acid, the stable manufactured form used in supplements and fortified foods. The body absorbs folic acid better than food folate: 240 micrograms of folic acid counts the same as 400 micrograms of food folate, which is why US labels give amounts in "dietary folate equivalents" (DFE).

Supplements also sell "methylfolate" (5-MTHF), the form the body uses directly, often marketed as better for people with a variant of the MTHFR gene. The US Centers for Disease Control and Prevention says that is not true. More than half of people in the US carry at least one copy of the common variant, and they can process folic acid; folic acid is the only form of folate shown in trials to prevent neural tube defects.

How it works

Folate carries the single-carbon units cells need to build DNA and divide, and — together with vitamin B12 — to recycle homocysteine into methionine. Tissues that divide quickly depend on it most: blood cells, and an embryo in its first weeks. Too little causes megaloblastic anaemia, in which red blood cells are large and immature.

The neural tube, which becomes the brain and spinal cord, forms and closes within the first month after conception — often before someone knows they are pregnant. That timing is the whole reason folic acid has to be taken before pregnancy rather than after a positive test.

What the research actually shows

Evidence strength varies by what is being claimed, so each use is graded separately against the published criteria rather than averaged into one reassuring letter.

Who should take it

Anyone who could become pregnant should take 400 micrograms of folic acid a day. The NHS advises starting before you stop using contraception — ideally three months before — and continuing until you are 12 weeks pregnant; if you find you are pregnant and have not been taking it, start straight away. The US Preventive Services Task Force recommends 400 to 800 micrograms a day for everyone planning to or able to become pregnant.

A higher dose of 5 mg a day, available on prescription, is advised if you or the baby's biological father have a neural tube defect or a family history of one, if you have had a previous affected pregnancy, if you have diabetes, sickle cell anaemia or thalassaemia, or if you take epilepsy or HIV medicines. Ask your GP.

Most other adults eating a varied diet do not need a folic acid supplement. From 13 December 2026, non-wholemeal wheat flour in the UK must be fortified with folic acid, which the government expects to prevent around 200 neural tube defects a year — about a fifth of the UK total. It is a safety net for unplanned pregnancies, not a substitute: the NHS advice to take a daily supplement continues.

Effective dosage

For pregnancy, 400 micrograms a day, or 5 mg on prescription for higher-risk pregnancies. In the Cochrane review, 400 micrograms worked as well as higher doses. Pregnancy multivitamins usually contain 400 micrograms — check the label, and avoid any that contain vitamin A (retinol), which the NHS advises against in pregnancy.

Adults need about 200 micrograms of folate a day from food in the UK, or 400 micrograms DFE in the US. The NHS says 1 mg or less a day of folic acid from supplements is unlikely to cause harm; the US upper limit is 1,000 micrograms a day from supplements and fortified foods combined. Folate that occurs naturally in food does not count towards the limit.

Where products diverge from the research. Most pregnancy supplements contain the right amount. The gap is timing: the neural tube closes within a month of conception, and the trials gave folic acid before pregnancy. A tablet started at the first scan arrives after the window it protects.

The other gap is marketing. Methylfolate products are sold as "active" or better absorbed, often at several times the price, and pitched at people with MTHFR variants. The CDC says people with those variants can use folic acid, and that folic acid is the only form of folate proven to prevent neural tube defects.

Safety, side effects and interactions

At the recommended amount, folic acid is safe: the CDC states that 400 micrograms a day has not been shown to cause harm.

Doses above 1 mg a day can hide a vitamin B12 deficiency. Folic acid corrects the anaemia that low B12 causes but not the nerve damage, which can progress unnoticed and become permanent — a particular concern for older people, whose B12 absorption falls with age.

Folic acid interacts with some medicines. It could interfere with methotrexate taken for cancer; anti-seizure medicines such as phenytoin, carbamazepine and valproate can lower folate, and folic acid can lower their blood levels; and sulfasalazine, used for ulcerative colitis, reduces folate absorption. Some people taking these medicines are prescribed folic acid deliberately, so follow your prescriber's advice.

None of this is medical advice. See the medical disclaimer for what that means before you act on anything here.