Quick answer
A pooled analysis of 13 randomized trials with 49,621 people found folic acid did not significantly change overall cancer incidence over about 5 years. NIH still says high doses might raise colorectal and possibly other cancer risk in some people, and that the effects of supplements are unclear. Ask your doctor if you have had colorectal polyps or cancer.1,2,3
Key facts
- A pooled analysis of 13 randomized trials with 49,621 people found no significant effect of folic acid on overall cancer incidence over about 5.2 years (risk ratio 1.06, range 0.99 to 1.13).1
- The same analysis found no significant effect on cancer of the large intestine, prostate, lung or breast.1
- NIH says high doses of folic acid might increase the risk of colorectal cancer and possibly other cancers in some people.2
- Folate that is naturally present in food is not harmful, and the upper limit does not apply to it.2
- A 2015 expert panel convened by the National Toxicology Program and NIH found folic acid supplements do not reduce cancer risk in people with adequate folate status.3
Does folate from food carry a cancer concern?
The concern is about folic acid, the form in supplements and fortified foods, and mainly at high doses. NIH's consumer fact sheet says folate that is naturally present in food and drinks is not harmful and that the upper limit does not apply to it.2
Folate that is naturally present in food and drinks is not harmful, and the upper limit does not apply to it.2,3
NIH's consumer sheet states the supplement concern in one sentence: high doses of folic acid might increase the risk of colorectal cancer and possibly other cancers in some people. The professional sheet says the effects of supplemental folic acid on cancer risk are unclear, especially for people with a history of colorectal adenomas or other cancers.2,3
What did the trials find?
The largest look at trials is a 2013 Lancet analysis. It pooled individual data from all 13 randomized trials of folic acid versus placebo that ran at least a year, had at least 500 participants and recorded cancer. Folic acid quadrupled blood levels, and over a weighted average of 5.2 years it had no significant effect on overall cancer incidence or on cancers of the large intestine, prostate, lung, breast or any other single site.1
Other trials and analyses that NIH summarizes point in different directions, which is why the table matters. Many of these trials gave folic acid with other B vitamins, which makes it hard to say what folic acid alone did.3
| Study | People | What it found |
|---|---|---|
| Pooled analysis of 13 randomized trials | 49,621 | No significant effect on overall or site-specific cancer incidence over about 5.2 years |
| Two Norwegian trials combined, in people with heart disease, 800 mcg folic acid plus 400 mcg B12 | 3,411 | Cancer incidence 21% higher and cancer deaths 38% higher than with no supplement, median 39 months |
| Fracture trial in adults 65 and older, 400 mcg folic acid with B12 and vitamin D3 | 2,919 | More cancers, especially colorectal and other digestive, than with vitamin D3 alone, over 2 years |
| Meta-analysis of 6 randomized trials in men | 25,738 | Prostate cancer risk 24% higher with folic acid |
| Pooled analysis of 3 adenoma trials, up to 3.5 years | Not stated | No effect on adenoma recurrence or on all cancers combined |
Why are the results mixed?
NIH's professional fact sheet describes a possible dual role for folate. Modest amounts taken before early tissue changes appear might suppress cancer development in healthy tissue, while high doses taken after such changes have started might speed their growth. NIH presents this as a hypothesis supported by some laboratory, animal and observational work, not as a settled finding.3
It also notes open questions: which dose and timing might have tumor-promoting effects, and whether any effect is specific to synthetic folic acid or to other forms of folate. The 2015 expert panel said consistent findings from human studies on folic acid and cancer growth justified more research.3
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.2,3
How does the upper limit relate to the cancer question?
NIH sets the upper limit for folate from supplements and fortified foods at 1,000 mcg a day for adults and says people should not exceed it unless a health care provider recommends it. The 2013 analysis found no significant effect at doses above those from fortification, and its authors noted that flour fortification gives doses roughly ten times smaller than the trial doses.1,2
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.2,3
Who should ask a doctor about folic acid and cancer?
- Anyone who has had colorectal adenomas, polyps or colorectal cancer, since NIH advises caution about amounts above the upper limit in that group.2,3
- Anyone being treated for cancer, especially with methotrexate, since folate supplements could interfere with its effects.2,3
- Men with a personal or family history of prostate cancer, since the pooled trial result in men is one of the findings above.3
- Anyone taking folic acid in amounts above the upper limit under a doctor's direction, who should keep that doctor informed of other supplements.3
Questions people ask
Does folic acid cause cancer?
A pooled analysis of 13 randomized trials found no significant effect on overall cancer incidence over about 5 years. NIH still says high doses might raise colorectal and possibly other cancer risk in some people. Ask your doctor if you have had polyps or cancer.1,2
Does folate cause colon cancer?
The 2013 pooled analysis found no significant effect on cancer of the large intestine. NIH says high doses of folic acid might increase colorectal cancer risk in some people, especially those with a history of adenomas. Ask your doctor.1,2,3
Is the folate in food a cancer risk?
NIH says folate that is naturally present in food and drinks is not harmful and that the upper limit does not apply to it.2
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Sources
- Vollset SE, Clarke R, Lewington S, et al. Effects of folic acid supplementation on overall and site-specific cancer incidence during the randomised trials: meta-analyses of data on 50,000 individuals. Lancet. 2013;381(9871):1029-1036. Opened 2026-10-11.
- National Institutes of Health, Office of Dietary Supplements. Folate: Fact Sheet for Consumers. Updated November 1, 2022. Opened 2026-10-07.
- National Institutes of Health, Office of Dietary Supplements. Folate: Fact Sheet for Health Professionals. Updated November 30, 2022. Opened 2026-10-07.
This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.