Vitamins During Cancer Treatment: Antioxidant Concerns and Talking to Your Oncology Team
What the National Cancer Institute, NCCIH and studies say about vitamin and antioxidant supplements during chemotherapy or radiotherapy, and why to tell your oncology team about everything you take.
Content last updated . This page has not been medically reviewed.
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If you are having or about to have cancer treatment, tell your oncology team about every vitamin and supplement you take, even ones unrelated to cancer. NCCIH (US) says to tell your health care providers about any complementary approach you use even if your reason for using it has nothing to do with cancer. [1] The National Cancer Institute (NCI) says cancer patients should inform their doctors about their use of any dietary supplement. [2] Nothing here is advice to stop or change your treatment.The concern in plain words
NCI explains that ionizing radiation kills cells by producing high levels of free radicals. [2] Antioxidants (such as vitamins A, C and E and carotenoids) are substances that counter free radicals, so the worry is that they might blunt a treatment. NCCIH says the effects of vitamin, mineral and antioxidant supplements during cancer treatment are uncertain, and that they might interfere with the effectiveness of chemotherapy or other therapies. [1] For antioxidant basics see Antioxidants explained.What research and agencies found
| Question | What the source found | Evidence label |
|---|---|---|
| Randomized trials of antioxidant supplements during treatment | NCI: results were mixed, but some found worse outcomes with supplements, especially in smokers. [2] | Mixed |
| Systematic review (2011) | The risk-benefit ratio of concurrent antioxidants with chemotherapy or radiotherapy is a controversial topic; more targeted studies are needed. [3] | Insufficient evidence |
| US breast cancer trial (SWOG S0221), 1,134 patients on chemotherapy, questionnaires about supplements | Any antioxidant supplement before and during treatment was associated with a higher hazard of recurrence (adjusted hazard ratio 1.41) that was borderline (P = .06); vitamin B12 use was associated with poorer disease-free survival; multivitamin use was not associated with survival. [4] | Observational |
| German breast cancer cohort (MARIE), 2,223 women | Concurrent antioxidant use with chemotherapy or radiotherapy among 1,940 women was associated with higher total mortality (hazard ratio 1.64) and worse recurrence-free survival. [5] | Observational |
| Laboratory studies | NCI: in some preclinical studies, antioxidants promoted tumor growth and metastasis in tumor-bearing mice. [2] | Preclinical |
Observational studies compare people who chose to take supplements with those who did not, so they cannot prove cause and effect; people who take supplements may differ in other ways. These findings are why the authors and agencies urge caution, not proof of harm for every product.
Proper evidence not found for large randomized trials testing a single named vitamin at a stated amount alongside a specific chemotherapy or radiotherapy regimen. I searched NCI, NCCIH and PubMed (the cohort studies and reviews above), and the 2011 review itself calls for such studies. [3]
Intravenous vitamin C
NCCIH says evidence on intravenous vitamin C and tumor growth or survival is mixed, with limited research in people, and that it appears well tolerated but carries serious toxicity risk for people with conditions such as iron overload or kidney disease. [1] NCI says the FDA has not approved IV vitamin C as a cancer treatment, and that it may cause serious side effects in people with kidney disease, G6PD deficiency or hemochromatosis. [6] See Intravenous vitamin C and cancer claims and IV vitamin therapy claims.Food versus supplements
This page covers supplements. Eating a varied diet during treatment is a topic for your oncology dietitian; the sources read here did not compare foods with supplements during treatment. For prevention trials see Vitamins and cancer prevention.Frequently asked questions
Can I take a multivitamin during chemotherapy?
Ask your oncology team. In the SWOG S0221 breast cancer study, multivitamin use was not associated with survival outcomes, but that is one observational study and does not replace your team's advice. [4]
Do antioxidants make chemotherapy less effective?
It is uncertain. NCCIH says supplements might interfere with treatment, and NCI says trials were mixed with some showing worse outcomes. [1][2]
Should I stop my supplements right away?
Talk to your oncology team promptly; they decide what suits your treatment. Do not stop prescribed cancer treatment.
Is vitamin C by IV a proven cancer treatment?
No. NCI says the FDA has not approved it as a cancer treatment, and NCCIH says evidence is mixed. [6][1]
What about vitamin B12 and iron?
In SWOG S0221, B12 and iron use during chemotherapy were associated with worse outcomes. These are associations in one study, so ask your team before taking either. [4]
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
How to read the evidence labels
Labels used on this page: mixed, insufficient evidence, preclinical, observational.
Safety and talking points
- Bring your supplement bottles, including multivitamins, and ask whether each is acceptable during your specific treatment.
- Ask about timing around chemotherapy cycles and radiotherapy, and about kidney and liver function if you take anything high dose. See Vitamins and kidney or liver disease.
- Ask about surgery plans too: Vitamins before surgery.
- Medicine interactions: Medication interactions; product quality: Quality testing and certifications.
- For marketing claims see Antioxidant safety and marketing claims.
References
- NCCIH (US National Institutes of Health). Cancer and Complementary Health Approaches: What You Need To Know. Last updated October 2021. Read 2026-10-22.
- National Cancer Institute (US). Antioxidants and Cancer Prevention. Read 2026-10-22.
- Nakayama A et al. Systematic review: generating evidence-based guidelines on the concurrent use of dietary antioxidants and chemotherapy or radiotherapy. Cancer Invest 2011;29(10):655-667. PubMed abstract. Read 2026-10-22.
- Ambrosone CB et al. Dietary supplement use during chemotherapy and survival outcomes of patients with breast cancer enrolled in a cooperative group clinical trial (SWOG S0221). J Clin Oncol 2020;38(8):804-814. PubMed abstract. Read 2026-10-22.
- Jung AY et al. Antioxidant supplementation and breast cancer prognosis in postmenopausal women undergoing chemotherapy and radiation therapy. Am J Clin Nutr 2019;109(1):69-78. PubMed abstract. Read 2026-10-22.
- National Cancer Institute (US). Intravenous Vitamin C (PDQ), Patient Version. Read 2026-10-22.