Can you get too much folate?

The upper limit of 1,000 mcg applies to synthetic folic acid only. No limit was set for folate from food, because high intakes from food have not caused harm.

What this means for you

The 1,000 microgram limit you will see quoted is not about spinach or lentils. It applies to folic acid, the synthetic form in supplements and fortified flour. No limit was set for folate from food, because high intakes from food have not been shown to cause harm.

That distinction matters because the harm from excess folic acid is indirect. It does not poison anyone. It hides something else.

Not for everyone. Large amounts of folic acid can correct the anaemia caused by vitamin B12 deficiency while the nerve damage underneath keeps progressing. The blood count improves and the person does not. This mostly concerns older adults, who are more likely to be low in B12 and more likely to be taking supplements, and it is the main argument for checking B12 before treating a low blood count as solved.

The one place extra folic acid has been measured doing damage

Not for everyone. If you are being treated for malaria with an antifolate drug, usually sulfadoxine-pyrimethamine, folic acid taken at the same time works against the treatment. Pooling eight trials in 3,486 people, treatment failure was about twice as likely at day 7 and still 35 per cent more likely at day 28, on moderate-certainty evidence. The parasites also cleared more slowly. Most of those trials used doses above the 1,000 microgram limit, so the caution belongs to high-dose folic acid during malaria treatment. If that is your situation, whoever is treating the malaria needs to know what you take.

A second finding rhymes with it. Women carrying twins who were given 4 to 5 mg a day had pre-eclampsia at 17.2 per cent against 9.9 per cent on placebo, and adjustment pulled the difference below significance without making it disappear. Neither of these proves that a large dose harms you. Together they are a good reason to stop assuming a large dose is free.

One other caution belongs to a specific group. People taking methotrexate for cancer should speak to their oncologist before taking folate, because it could interfere with how the drug works. That is a different situation from the low-dose folic acid often prescribed alongside methotrexate for arthritis, which is a decision their own doctor has already made.

What about unmetabolised folic acid

This is the worry most people arrive with now, and it deserves a straight answer rather than either reassurance or alarm.

Unsettled. The observational case against unmetabolised folic acid does not hold together. A systematic review found the associations inconsistent, confounded and never replicated, and it named the reason they cannot be cleaned up: folic acid raises unmetabolised folic acid and total folate together, so in observational data nobody can tell which one they are looking at. Measurement makes it worse. Unmetabolised folic acid is at most 1 to 2 per cent of the folate in your blood, and one reading of it says as much about when you last swallowed a tablet as about your dose. The same reviewers are careful to call this an absence of a reliable signal rather than a demonstration of safety. Their conclusion is that the evidence cannot support a claim in either direction.

Methylfolate is sold as the answer to that worry, and it does the narrow thing it promises. Pooling three trials, swapping in an equimolar dose of methylfolate lowered plasma unmetabolised folic acid, on evidence GRADE rates as low certainty. In pregnancy the swap was tested directly: 7 per cent of mothers on methylfolate had detectable unmetabolised folic acid against 31 per cent on folic acid, and only two cord samples had any, both from the folic acid arm. The doses were not matched, 1,000 against 1,330 micrograms of dietary folate equivalents, so part of that difference is dose rather than form.

The other half of the same trial is the part that gets left out of the marketing. After 24 weeks there was no difference between the two forms in total folate in the mother's blood, in the cord blood or in the placenta. Sixty-two women completed it and no clinical outcome was measured at all. Lowering a blood marker is a different achievement from helping a baby, and only the first one has been shown.

For most people, the fortified bread and the bowl of lentils are not a problem and were never meant to be one.

One thing to keep in view while reading this page: the deficiency it protects against is not subtle. The main sign is megaloblastic anaemia, where red blood cells are large and abnormal, with weakness, fatigue, poor concentration, headache and breathlessness, and a sore tongue and mouth ulcers alongside.

The reason folic acid is treated carefully at high doses is that it works so well at low ones. In trials, taking folic acid before and in early pregnancy reduced neural tube defects, which is why the 400 microgram recommendation exists and why fortification was introduced in the first place. A deliberate, modest dose with a clear purpose is a different thing from a high-dose supplement taken indefinitely, and the upper limit applies only to the synthetic form.

Two findings sharpen the case for the recommended dose rather than a larger one. In a meta-analysis of 39 studies of children, mothers taking far more folic acid than recommended saw no improvement in their children's development and possibly the reverse. And in people who have already had a bowel polyp removed, folic acid supplements did not reduce recurrence, with a relative risk of 1.05 on high-quality evidence. More is not the direction the evidence points. the full review is here.

Written from the evidence cards and the data on this page, then checked back against the sources. Bioma Learn has no human medical reviewer at this time and we say so rather than invent one. Information, not medical advice. How we verify.

Upper limits apply to everything you take in, from supplements and, for most nutrients, from food as well. Reaching one from whole food alone is uncommon. Supplements are where people cross the line, usually without meaning to.

Full guide: Folate and folic acid: the clearest success in nutrition policy, and its one side effect