Folic acid 0.4 mg

Taken from before conception, it prevents most neural tube defects. Outside pregnancy, Canadians already get plenty.

At a glance

Our verdict	Before and in early pregnancy. A previous neural-tube defect needs a higher prescription dose.
Best use	Before and in early pregnancy
How sure the evidence is	High for preventing neural tube defects
Cost a month	Not costed in our review

Written for adults in Canada. It explains the evidence; it is not advice for you personally. Last reviewed 20 October 2026.

Supplement Check

Is it right for you?

The right folic acid dose depends on your own and your partner's history, diabetes, weight, seizure medicines, and what else you already take. Some people need a higher, prescribed dose, and we find them. Our Supplement Check looks at:

Whether you are planning a pregnancy
Higher risk, a higher prescribed dose: a previous pregnancy affected by a neural tube defect, you or the baby's father born with one, diabetes, a body mass index of 30 or more, anti-seizure medicines, or methotrexate
Folic acid over 1 mg a day that a doctor did not prescribe
Haemochromatosis or iron overload
Current smokers, people who quit 10 years ago or less, or anyone with a history of lung cancer
Every medicine you take
The total across everything you already take, against the safe upper limits

Coming soon. The Supplement Check and the Online Health Assessment open once our safety testing is complete.

$59 when it opens, paid once. Adults 18 and over, anywhere in Canada. While the service is new, Dr. French reads every report himself, and your report arrives within 3 working days. More about the Supplement Check

Tell me when it opens

What it is

Folic acid is the synthetic form of folate, a B vitamin found in leafy greens, legumes and fortified grains. Since 1998 Canadian flour and pasta have been fortified with it.

We suggest it inside one multivitamin containing folic acid 0.4 mg (400 micrograms); we have not named the product yet. Some multivitamins use L-methylfolate instead, and that is not the product we use for planning a pregnancy. Folic acid above 1 mg a dose is prescription-only in Canada. [1]

Structure of Folic acid 0.4 mg — Structure: PubChem (public domain).

How it acts

Folate works with vitamin B12 in the cycle that turns homocysteine into methionine; a raised homocysteine is a sign the body is short of B12 or folate. Our sources give the size of folic acid's effect on neural tube defects, not how it prevents them.

Outline of a person showing where Folic acid 0.4 mg acts in the body

What it is used for, and what the evidence says

Dr. French's ruling: folic acid only for pregnancy, planned or under way.

Use	What the trials found	How sure we are
Preventing neural tube defects	A Cochrane review of folate taken around conception: neural tube defects cut sharply (relative risk about 0.31). [2]	High
After a pregnancy affected by a neural tube defect, 4 mg a day	The Medical Research Council trial: recurrence cut by 72%. [3]	High
Fortification in Canada	Neural tube defects fell by 46%, from 1.58 to 0.86 per 1,000 births. [4]	High
Cancer	13 trials, 49,621 people over 5.2 years: no overall effect on cancer (relative risk 1.06). [5] Single trials raised concerns: folic acid with B12 in unfortified Norway (21% more cancer), [6] and 1 mg in a bowel-polyp trial (more advanced polyps at the second check). [7] A promoting effect on growths already present is not excluded.	Low
Stroke	In China, where flour is not fortified, folic acid added to enalapril cut strokes (hazard ratio 0.79). [8] That does not carry over to Canada.	Low
Hiding a B12 shortage	Folic acid corrects the anaemia of B12 deficiency while the nerve damage goes on. In older Americans after fortification, a low B12 with a high folate went with anaemia (odds ratio 3.1) and impaired thinking (2.6). [9]	Low

How sure we are: High means further research is very unlikely to change the answer; Moderate, it might; Low, it may well change; Very low, we are very unsure. These follow the GRADE method that guideline panels use.

Benefits

Prevents most neural tube defects (spina bifida, anencephaly) when taken from before conception (high certainty).
A higher prescribed dose cuts recurrence by about 72% after an affected pregnancy.
Outside pregnancy in Canada: no benefit shown, and most people already have plenty.

Risks and side effects

Hiding a B12 shortage at high doses: the blood count improves while nerve damage continues. [9]
A possible effect on growths already present at higher doses, not excluded (low certainty). [5, 7]
Going over the upper limit: 1,000 micrograms a day of synthetic folic acid from supplements and fortified food. [10] One common prenatal vitamin reaches it on its own, and fortified flour and pasta add more.
Vitamin A: a higher dose is taken as a separate folic-acid tablet, never as extra multivitamins.

Interactions with medicines

Our sources list no effect of folic acid on how the liver clears medicines. Some medicines work against folate, so people taking them need a higher, prescribed dose.

Medicine	What can happen	What to do
Anti-seizure medicines	Raise the folic acid needed in pregnancy	Higher dose, prescribed by a doctor; Dr. French reviews
Methotrexate and other medicines that block folate	Work against folate	Prescribed folic acid is counted, never stopped; a higher dose is for the prescriber

Who should not take it

Adults not planning a pregnancy (Dr. French's ruling): 40% of Canadians already have high folate levels.
Higher risk, a higher prescribed dose: a previous pregnancy affected by a neural tube defect, you or the baby's father born with one, diabetes, a body mass index of 30 or more, anti-seizure medicines, or methotrexate. Book a consultation; until then keep taking one multivitamin with 0.4 mg.
Folic acid over 1 mg a day that a doctor did not prescribe: flagged, with a B12 check first.
Haemochromatosis or iron overload: an iron-free product only.
Current smokers, people who quit 10 years ago or less, or anyone with a history of lung cancer: a multivitamin without beta-carotene.
An overactive thyroid, thyroid nodules, goitre or thyroid cancer: no multivitamin from the check.
Under 18: outside the Supplement Check.

Dose and timing

0.4 mg (400 micrograms) a day, in one multivitamin, from at least 3 months before pregnancy, and on through early pregnancy.
One multivitamin only, with no more than 10,000 IU of vitamin A as retinol.
After a pregnancy affected by a neural tube defect: 4 mg a day by prescription, from at least 3 months before conception to 12 weeks, as a separate folic-acid tablet, alongside one standard prenatal multivitamin. [3, 11]
Count every product against the 1,000 microgram upper limit.
Food first: leafy greens, legumes and fortified grains, alongside the multivitamin.
How to tell it is working: our review gives no test to follow; what counts is starting before conception.
Store at room temperature.

What we know about Canadians

Since flour and pasta were fortified with folic acid in 1998, folate deficiency has all but disappeared in Canada: under 1% are deficient, and 40% have a high red-cell folate. [12] Neural tube defects fell by 46%, [4] but the rate has since crept up, from 3.6 to 4.6 per 10,000 births between 2004 and 2015, partly because more mothers have diabetes before pregnancy. [13] The gap is in planning: 22% of women of childbearing age are below the folate level that gives the most protection, [12] only 16.5% of women 15 to 55 who are not pregnant take a folic acid supplement, and 64% of mothers took one in the 3 months before pregnancy. [14]

Most at risk: women who could become pregnant and are not taking folic acid (more often younger, single, with lower education or income, or smokers), and women with diabetes before pregnancy, obesity, or on anti-seizure medicines.

How sure are we: high (deficiency, fortification); moderate (the rest). Measured national survey (2007 to 2009) and population birth data; the folate test method changes the numbers.

An honest comparison: neural tube defects

The same outcome. Our review did not cost folic acid on its own.

Option	Effect	How sure we are	Cost a month (CAD)
Folic acid 0.4 to 1 mg from before conception	Neural tube defects cut sharply (relative risk about 0.31) [2]	High	Not costed in our review
Fortified food (flour and pasta, since 1998)	46% fewer neural tube defects across Canada [4]	High	Already in your food
Folic acid 4 mg by prescription, after an affected pregnancy	72% fewer recurrences [3]	High	Not costed in our review

Fortification protects everyone a little; 0.4 mg taken from before conception protects the person who takes it. The planned dose is still needed.

Our verdict

For anyone who could become pregnant or is planning a pregnancy: one multivitamin with folic acid 0.4 mg a day, from at least 3 months before conception. A higher dose is prescribed by a doctor for those at higher risk, and Dr. French reviews every pregnancy report.

Not for adults who are not planning a pregnancy.

Folic acid before conception is one of the few supplements with high-certainty evidence against a serious outcome. Outside pregnancy the picture is different: fortification means 40% of Canadians already have high folate, extra folic acid has not been shown to help them, and at high doses it can hide a B12 shortage. Dr. French's ruling is folic acid only for pregnancy, at 0.4 mg, with the higher prescribed dose for those who need it.