Folic Acid

Is folic acid safe?

Yes, at the doses most people take, and for women who could become pregnant it is one of the few supplements health agencies actively recommend. A daily 400 mcg tablet, started before conception and taken to week 12, sharply cuts the risk of spina bifida and other neural tube defects. Trials found 60% to 100% fewer cases.

The cautions start above 1,000 mcg (1 mg) a day, the upper limit for adults in the US and confirmed by the European Food Safety Authority in 2023. At high doses, folic acid can hide a vitamin B12 deficiency, and a few trials suggest it may speed up existing bowel polyps. Folate from food carries none of these concerns; the limit applies to folic acid and other added forms only.

Why pregnancy is the main reason to take it

The neural tube closes between the 21st and 28th day after conception, often before a woman knows she is pregnant. That is why advice is to start early. Government advice in the UK is 400 mcg a day from the time you start trying, ideally three months before, until 12 weeks. Women at higher risk, for example with a previous affected pregnancy, are advised to take 5 mg a day; the NHS says this is unlikely to cause harm because it is short-term, but it should be agreed with a doctor first.

Fortification shows the benefit at population scale. After the US made folic acid fortification of enriched grain mandatory in 1998, neural tube defects fell by about 30%. More than 60 countries now add folic acid to flour. The UK will require 0.25 mg per 100 g in non-wholemeal wheat flour from December 2026, and the government expects about 20% fewer neural tube defects as a result. Fortification does not replace the supplement for women planning a pregnancy.

How folic acid can hide a B12 deficiency

Low vitamin B12 and low folate both cause the same kind of anaemia, with large, immature red blood cells. Folic acid fixes that anaemia even when the real problem is B12. The blood test looks better while B12 deficiency keeps damaging the nerves, sometimes permanently. According to the Linus Pauling Institute, cases of nerve damage progressing this way have mostly involved doses of 5 mg and above.

This is why the 1,000 mcg upper limit exists, and why it matters most for people over 50, vegans, people on metformin or long-term acid-reducing drugs, and anyone with numbness or tingling. If you are in one of these groups, ask for a B12 test before taking high-dose folic acid.

Does high-dose folic acid raise cancer risk?

The evidence is mixed but worth knowing. In a US trial published in JAMA in 2007, 1,021 people with a history of bowel polyps took 1 mg of folic acid or a placebo. Folic acid did not prevent polyps. At the second check, advanced polyps were more common in the folic acid group (11.6% against 6.9%), as were three or more polyps and cancers outside the bowel. In the Dutch B-PROOF trial of 2,524 older adults, folic acid 400 mcg with B12 for two to three years was linked to more colorectal cancer, though cancer was not the trial's main outcome. A pooled analysis of three large polyp trials found no overall increase.

The working view is that folate helps prevent cancer from starting but that large doses might feed growths already present. It is a reason not to take high-dose folic acid without a clear need, not a reason to avoid the pregnancy dose.

Medicines that interact

• Methotrexate: at low doses for arthritis or psoriasis, doctors prescribe folic acid to reduce side effects. In cancer treatment, folic acid works against methotrexate and other antifolate drugs such as pemetrexed. Follow your prescriber's instructions exactly.
• Anti-seizure drugs (phenytoin, phenobarbital, primidone) lower folate levels, and changing your folic acid dose can affect seizure control. Women on these drugs who plan a pregnancy need specialist advice on dose.
• Sulfasalazine, trimethoprim, pyrimethamine and triamterene have antifolate effects.
• Cholestyramine and colestipol reduce absorption; take folic acid at a different time of day.

Who should be careful

Talk to your doctor or pharmacist before taking more than 400 mcg a day if you are over 50, vegan, have had bowel polyps or bowel cancer, take any of the medicines above, or have unexplained numbness, tingling or memory problems. Check the label too: many multivitamins and B-complex products already contain 400 mcg or more, and fortified cereals add to it. Pairing a prenatal product with a separate folic acid tablet is usually unnecessary unless you have been prescribed the 5 mg dose.

How we scored it

Regulatory record (95) is the strongest factor: governments add folic acid to flour rather than restrict it. Dose safety (85) and quality (85) are good, and organ toxicity (82) loses points for the polyp and cancer signals at high doses. Interactions (76) is the weakest factor because of methotrexate, anti-seizure drugs and B12 masking. Folic acid sits close to vitamin D and well above vitamin A, the other vitamin with a pregnancy warning, but for the opposite reason.