Evidence Explorer · MS-N227 · Supplements and cancer prevention · Appraised November 3, 2026
One milligram of folic acid daily did not prevent recurrent colorectal adenomas and produced concerning later signals for advanced and multiple adenomas. The findings do not apply to treating a documented folate deficiency at a clinician-directed dose.
Evidence at a glance
Benefit signal: No adenoma-prevention benefit
Certainty: Moderate for no benefit; possible harm signal requires caution
Risk context: High-dose folic acid can mask vitamin B12 deficiency and may be inappropriate after adenoma removal
What was studied
A double-blind phase III trial randomized 1,021 adults with a recent adenoma to folic acid 1 mg/day (n=516) or placebo (n=505), alongside a separate aspirin randomization. Surveillance covered two colonoscopy cycles.
At three years, adenoma incidence was 44.1% versus 42.4% (RR 1.04, 95% CI 0.90–1.20). In later follow-up, advanced lesions were 11.6% versus 6.9% (RR 1.67, 95% CI 1.00–2.80), and folic acid was associated with more participants having at least three adenomas.
What this evidence cannot establish
Adenomas are precursors, not invasive colorectal cancer outcomes. Later results included fewer participants and some harm estimates were borderline.
The trial tested 1 mg/day supplementation after adenoma removal, not usual food folate or treatment of deficiency.
Practical safety context
Folic acid can correct anemia while allowing neurologic injury from vitamin B12 deficiency to progress. Review B12 status, total intake from fortified foods and multivitamins, and the indication for supplementation.
Do not use folic acid as a substitute for guideline-based surveillance colonoscopy.
Questions for the care team
- Do I have a confirmed folate deficiency or another clear indication?
- How much folic acid am I already getting from fortified foods and supplements?
- Is my vitamin B12 status known, and when is surveillance colonoscopy due?
Sources
- Randomized folic-acid adenoma-prevention trial (PMID 17551129)
- Trial registration (NCT00272324)
Sources and publication status checked November 3, 2026. The later advanced-lesion and multiplicity signals are reported as possible harm, not proof of increased invasive cancer.
For education, not personal medical advice. Nutrition and supplement decisions during cancer care should be individualized with the oncology and dietetics teams.
