Folate
Also called: folic acid, vitamin B9.
Last reviewed 2026-09-16. How this works.
When to call your team — there is nothing to watch for
Whatever you are taking or thinking of taking, the people treating you can only weigh it if they know about it. That is true when nothing has been written down here, and it is most true then.
The harm recorded on this page is a raised chance of developing cancer over years. It is not something a person can feel happening, so there are no signs to watch for, and that is the point rather than an omission. What it is a reason for is telling your team what you take.
This block is about what to watch for, and it is not all this page holds: what has been recorded about harm is below. Nothing in this block is an all-clear for a question it does not mention.
Preventing colorectal adenomas Grade E
Human evidence points the wrong way.
Harm has been recorded in people for this claim. The grade measures how strong the evidence for benefit is; it says nothing about the harm, and the two are read from different parts of this page.
In a trial to see if folic acid stopped bowel growths, the group taking it had more of the worrying ones.
The folic acid arm had more advanced lesions, and more people with several of them.
The harm
A double-blind placebo-controlled trial randomised 1,021 people with a recent history of bowel adenomas. Folic acid did not reduce adenoma risk. At the second follow-up, at least one advanced lesion was found in 11.6 percent of the folic acid group against 6.9 percent on placebo, a relative risk of 1.67.1
Having three or more adenomas was more than twice as common in the folic acid group, 9.9 percent against 4.3 percent, and there were more non-colorectal cancers. A later report from the same trial found increased prostate cancer risk.1
The trial authors concluded that folic acid does not reduce adenoma risk and that further research is needed into the possibility that it increases the risk of colorectal neoplasia. That is this book's definition of an E.1
The claim
The claim was that folic acid prevents bowel growths from forming, and by extension prevents bowel cancer.
What it does
Reviews describe folate as a dual modulator: moderate increases before abnormal cells are established may be protective, while excess, or an increase after early lesions already exist, may promote growth. That account is the position of reviews and is described there as corroborated by animal work rather than established in people.3
The evidence
The scope of this grade matters and the book states it explicitly. The E applies to supplementing people who are already replete, especially those who already have lesions. It is not a statement about folate in food, and the pregnancy indication is excluded from it entirely.13
A separate randomised trial found the opposite in the people the deficiency argument is actually about. Overall it found no significant effect, but among participants with low plasma folate at baseline, those given folic acid had significantly fewer recurrent adenomas.2
That is the fourth independent instance of the same principle in this chapter, after selenium, vitamin D and vitamin C. What a supplement does depends on whether the person taking it was short of it.2
The studies this rests on
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Folic acid for the prevention of colorectal adenomas (NCT00272324)1
A randomised trial, double blind, measured a clinical outcome, 1021 participants.
no reduction in adenoma risk; more advanced lesions and more multiple adenomas in the folic acid group.
relative risk 1.67, interval 1 to 2.8. at least one advanced lesion, 11.6 percent against 6.9 percent, P=0.05
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Folic acid supplementation and recurrent colorectal adenoma2
A randomised trial, double blind, measured a clinical outcome, 672 participants.
no significant effect overall; fewer recurrent adenomas among those with low plasma folate at baseline.
relative risk 0.82, interval 0.59 to 1.13. overall; low baseline folate subgroup 0.61, interval 0.42 to 0.90
What to ask
Is folic acid something I should be taking, given my history?
The dish-to-person gap
The register holds no concentration comparison for folate.
Interactions
The register holds no measured interaction between folic acid and a cancer treatment class. Chapter 16 of the book covers supplement interactions in general, and an empty list here means nobody has measured this one.
Nobody having looked is not the same as nothing being there. That is what this line says and all it says.
The whole interaction list, and how to turn it into a pharmacy review request.
Find a trial instead
This question has been answered by randomised trials.
References
Cole BF, Baron JA, Sandler RS, Haile RW, Ahnen DJ, Bresalier RS, McKeown-Eyssen G, Summers RW, Rothstein RI, Burke CA, et al, Polyp Prevention Study Group. Folic acid for the prevention of colorectal adenomas: a randomized clinical trial. JAMA 2007;297(21):2351-2359.
doi:10.1001/jama.297.21.2351 · NCT00272324
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 17 note 13. The author string is abbreviated in the source that was read and has not been expanded.
A randomized trial on folic acid supplementation and risk of recurrent colorectal adenoma. American Journal of Clinical Nutrition, 2009.
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 17 note 15.
The dual-modulator account of folate in colorectal carcinogenesis, set out in reviews of folate and colorectal cancer prevention.
No persistent identifier recorded.
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 17 note 14.