# Folate and folic acid supplements

Source: https://onco.cc/technologies/folic-acid-supplements/  
OnCo record `folic-acid-supplements` (Technology). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

Folic acid is important in pregnancy, but during cancer treatment it can work against some drugs: it can blunt methotrexate used to treat cancer and has been linked to more severe side effects with capecitabine. It is not the same as the folinic acid (leucovorin) that oncologists deliberately give with some chemotherapy. Tell your team before taking it.

## Summary

Folate is needed for DNA synthesis and cell division, which is exactly why several cancer drugs target folate metabolism. Memorial Sloan Kettering's About Herbs entry lists the interactions that matter in oncology: folic acid can decrease the efficacy of methotrexate used to treat cancer, including acute lymphoblastic leukaemia; with capecitabine, high folic acid intake produced severe adverse reactions in two case reports, and folic acid-containing supplements were associated with more capecitabine toxicity in an observational study. Evidence on folate and cancer risk is mixed: higher dietary folate may be associated with lower breast and pancreatic cancer risk, studies of supplements and colorectal cancer are mixed with some suggesting harm, and folic acid combined with other B vitamins showed no effect on breast or invasive cancers and possibly more cancer in people with heart disease. One case report associated folate and thiamine with lymphoma progression. Excess folic acid can also mask vitamin B12 deficiency. Folinic acid (leucovorin) given with fluorouracil, or as rescue after high-dose methotrexate, is a prescribed part of those regimens and should not be confused with over-the-counter folic acid. Folic acid before conception and in early pregnancy reduces neural tube defects and is standard advice.

## Fields

- Kind: Technology
- Status: established
- Last checked: 2026-11-04
- Also known as: Folic acid; L-methylfolate; Vitamin B9
- Tags: complementary; supportive-care; evidence:harm
- Principle: Methotrexate, pemetrexed and fluoropyrimidines act on folate-dependent enzymes; extra folate can bypass the block (methotrexate) or deepen the binding of fluorouracil's metabolite to its target (capecitabine), changing efficacy or toxicity.
- Strengths: Prevents neural tube defects in pregnancy; Corrects megaloblastic anaemia due to deficiency
- Limitations: Can reduce the efficacy of methotrexate; Associated with more severe capecitabine toxicity; Mixed and partly unfavourable cancer risk data for supplements

## Sources

- MSK About Herbs: Folate (page last updated 27 August 2024): https://www.mskcc.org/cancer-care/integrative-medicine/herbs/folate

## Connected records

- technologies: [Dietary supplements during cancer treatment: interactions and harms](https://onco.cc/technologies/dietary-supplements-treatment-interactions/), [Evidence-based integrative oncology](https://onco.cc/technologies/integrative-oncology/), [Vitamin B12 supplements](https://onco.cc/technologies/vitamin-b12-supplements/), [Vitamin B6 (pyridoxine) for hand-foot syndrome and other uses](https://onco.cc/technologies/vitamin-b6-pyridoxine/)
- fronts: [Chemotherapy](https://onco.cc/fronts/chemotherapy/), [Diet, Exercise & Lifestyle](https://onco.cc/fronts/nutrition-lifestyle/), [Supportive Care & Survivorship](https://onco.cc/fronts/supportive-care/)
- drugs: [Capecitabine](https://onco.cc/drugs/capecitabine/), [Fluorouracil (5-FU)](https://onco.cc/drugs/fluorouracil/), [Leucovorin (folinic acid)](https://onco.cc/drugs/leucovorin/), [Methotrexate](https://onco.cc/drugs/methotrexate/)
- bottlenecks: [Toxicity and quality of life are undervalued](https://onco.cc/bottlenecks/b-toxicity-qol/)

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