Quick answer

At or below the adult upper limit of 1,000 mcg a day from supplements and fortified foods, folic acid rarely causes side effects. Allergic reactions are rare, and stomach upset was reported at 15 mg a day. The main concern with large amounts is that they can hide low vitamin B12.^1,2,3

Key facts

Folate that occurs naturally in food has no upper limit.¹
The stomach and sleep effects on the folic acid label were seen at 15 mg a day, 15 times the adult upper limit.^1,2
The upper limit covers folic acid in fortified foods as well as in supplements.¹
The prescribing label calls folic acid relatively nontoxic in people.²

Side effects reported

Allergic reactions to folic acid are rare. Reported signs include skin redness, rash, itching, feeling generally unwell and difficulty breathing from narrowed airways.²

In people given 15 mg (15,000 mcg) of folic acid a day for a month, reported effects included loss of appetite, nausea, bloating, gas, a bitter taste, altered sleep, difficulty concentrating, irritability, excitement and confusion.²

Those stomach, sleep and mood effects came from a high-dose treatment setting. At supplement amounts, the label describes folic acid as relatively nontoxic.²

The main risk: hiding low vitamin B12

Large amounts of folic acid can correct the anemia caused by low vitamin B12 without stopping the nerve damage it also causes, so the B12 problem can go unnoticed until the damage is permanent. Large doses might also make the effects of low B12 worse.^1,2,3

That is why the folic acid label advises against more than 400 mcg a day, outside pregnancy and breastfeeding, until pernicious anemia has been ruled out. Low B12 is more common after 50 and in people who eat no animal foods.^2,4

The folic acid prescribing label notes that blood vitamin B12 levels may fall during prolonged folic acid therapy.²

Other concerns with high doses

High doses of folic acid might increase the risk of colorectal cancer and possibly other cancers in some people, so NIH advises caution about amounts above the upper limit, especially for people who have had colorectal adenomas.^1,3

The evidence is mixed. In a 2-year trial in 2,919 adults aged 65 and over, 400 mcg of folic acid with 500 mcg of B12 and vitamin D was followed by more cancers, mainly colorectal and other gut cancers, than vitamin D alone. A pooled analysis of 13 trials, with a median of 2,000 mcg a day for about 5 years, found no significant effect on overall cancer rates.³

In one study NIH cites, mothers who took 1,000 mcg or more of folic acid a day from supplements around conception had children with lower scores on several development tests at age 4 to 5 than mothers who took 400 to 999 mcg.³

Folic acid beyond what the body can convert circulates unchanged. It has been linked to lower numbers and activity of natural killer cells, part of the immune system, but whether that harms health is not known.³

Is methylfolate safe?

NIH sets no separate limit for methylfolate, and its consumer fact sheet says not to take folate from supplements or fortified foods above the upper limit unless a health care provider recommends it. The B12 masking concern and the medicine interactions apply to folate in general.^1,3

In one small study, methylfolate rarely left unchanged folic acid in the blood. That has not been shown to make high doses of it any safer.⁵

Folic acid in older adults

Among adults, those aged 51 to 70 are the most frequent users of folic acid supplements, and about 5% of them, along with men over 70, go over the upper limit. Older adults are also more likely to have low vitamin B12, which large amounts of folic acid can hide.^3,6

Medicines that interact with folate

Ask your pharmacist or doctor first

Folate supplements could interfere with methotrexate when it is used for cancer, and the methotrexate label says to take products containing folic acid then only if a doctor directs it. When methotrexate is used for rheumatoid arthritis, psoriasis or polyarticular juvenile idiopathic arthritis, the same label tells prescribers to give folic acid or folinic acid to reduce side effects.^1,3,7,8

On the medicine's label

Ask your pharmacist or doctor first

Phenytoin, carbamazepine and valproate can lower blood folate, and folate supplements might lower blood levels of these medicines. The phenytoin and folic acid labels both say folic acid can lower phenytoin levels or weaken its effect.^1,2,3,9

On the medicine's label

Ask your pharmacist or doctor first

Sulfasalazine reduces how much folate the body absorbs and can lead to low folate. Its label reports reduced absorption of folic acid when the two are taken together.^1,3,10

On the medicine's label

Who should ask first

The upper limit for adults is 1,000 mcg a day of folate from supplements and fortified foods, which for folic acid equals about 1,667 mcg DFE. It is 800 mcg for teens aged 14 to 18. Folate naturally present in food has no limit, and the limits do not apply to higher doses taken under medical supervision.^1,3

Anyone taking methotrexate, an anti-seizure medicine or sulfasalazine
Anyone with pernicious anemia or low vitamin B12, or who plans more than 400 mcg of folic acid a day without a B12 check²
Anyone who has had colorectal adenomas or colorectal cancer
Anyone pregnant or planning a pregnancy who is unsure how much to take

Questions people ask

Can you take too much folate?

From food, no. From supplements and fortified foods, adults should stay at or under 1,000 mcg of folic acid a day unless a doctor advises more, and teens aged 14 to 18 at or under 800 mcg.¹

Does folic acid cause different side effects in women?

The sources we use do not report different side effects by sex. For women, the extra caution is pregnancy: very high doses around conception were linked in one study to lower test scores in children, so keep to the advised amount unless a doctor sets a higher one.³

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Sources

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.