Antioxidants During Chemotherapy and Radiation: What the Research Shows

Whether to take antioxidants during chemotherapy and radiation is one of the most common questions people with cancer ask, and opinions on both sides are strong. The real answer depends on which antioxidant, how much, when, and with which treatment. This article explains the science behind the debate, what the randomized trials show, which antioxidants have the best evidence, and which to avoid during treatment.

Part of our guide: Natural Compounds and Cancer: What the Human Studies Show

Why the Question Matters

Radiation and many chemotherapy drugs kill cancer cells partly by creating free radicals, unstable molecules that damage cells. The concern is that antioxidants could neutralize those free radicals and blunt treatment. The other side of the argument is that antioxidants can protect healthy tissue, reduce side effects, and help people complete their full treatment. Some antioxidants, such as high-dose intravenous vitamin C, also act very differently at high doses, generating free radicals inside cancer cells rather than removing them.

What the Randomized Trials Show

A review of 19 randomized trials including 1,554 patients found no trial in which antioxidants reduced the effectiveness of chemotherapy. Many of the trials found fewer side effects, and some found better tumor response or survival.¹

High-dose IV vitamin C. In a University of Kansas trial of women with ovarian cancer, IV vitamin C with chemotherapy reduced side effects.² In the Chinese VITALITY trial of 442 patients with metastatic colorectal cancer, led from Sun Yat-sen University Cancer Center, high-dose IV vitamin C added to FOLFOX chemotherapy was safe and did not weaken chemotherapy, and patients whose tumors carried a RAS mutation appeared to benefit.³ See our article on high-dose IV vitamin C.

Melatonin. A powerful antioxidant hormone, melatonin reduced chemotherapy side effects across many randomized trials. See our article on melatonin.

The Studies That Raise Caution

Breast cancer, SWOG S0221. In a large American breast cancer chemotherapy trial, women who reported taking antioxidant supplements, including vitamins A, C and E, carotenoids or coenzyme Q10, both before and during chemotherapy had a 41 percent higher risk of their cancer returning, a result that fell just short of statistical certainty. Vitamin B12 use was linked to worse outcomes.⁴ This was not a randomized test of antioxidants, but it is the largest study of supplement use during chemotherapy.

Head and neck cancer, Canada. In a Canadian trial, people receiving radiation for head and neck cancer who took synthetic vitamin E, alpha-tocopherol alone, had more second cancers during the period they took it.⁵ The form matters: synthetic alpha-tocopherol alone is not the same as the mixed natural forms of vitamin E.

Laboratory studies from Sweden. At the University of Gothenburg, researchers found that the antioxidants N-acetylcysteine (NAC) and vitamin E made lung tumors in mice grow faster,⁶ and that NAC increased the spread of melanoma.⁷ Cancer cells use their own antioxidant system, built around glutathione, to protect themselves, and NAC feeds that system.

What This Means in Practice

NAC and glutathione. Our book advises against NAC and glutathione during cancer treatment, because they directly strengthen the defense tumors use to resist chemotherapy. The only exception is a short course for a severe, acute problem.

Vitamin E. If vitamin E is used, it should be the natural mixed tocopherols and tocotrienols, never synthetic alpha-tocopherol alone.

Food. Antioxidants from vegetables, herbs and spices are not a concern and are part of a healthy diet during treatment. While cancer is active, fruit is best limited because of its natural sugar content. Small amounts of lower-sugar fruits, such as berries, are the better choice.

Timing. Many integrative practitioners use antioxidant supplements intermittently, away from the days chemotherapy or radiation is given, to protect healthy tissue without shielding the tumor while treatment is active.

Interactions. High-dose vitamin C and green tea extract can block the myeloma drug bortezomib. See our articles on multiple myeloma and combating chemotherapy side effects.

How Cancer Expert MD Helps

Cancer Expert MD provides education and consultation for people with every type and stage of cancer, and specializes in Stage IV and metastatic cancer. Our book, The New Reality of Cancer, has a full chapter on antioxidants in cancer care, covering each antioxidant, how it works, what studies in people show, and its drug interactions.

Frequently Asked Questions

Do antioxidants interfere with chemotherapy?

A review of 19 randomized trials found none in which antioxidants reduced chemotherapy’s effectiveness, and many found fewer side effects. A large breast cancer study found more recurrences in women taking antioxidant supplements both before and during chemotherapy, so the choice of antioxidant and timing matter.

Should I take NAC or glutathione during treatment?

Our book advises against them during treatment, because they strengthen the glutathione defense tumors use to resist chemotherapy, except for a short course for a severe, acute problem.

Is IV vitamin C safe with chemotherapy?

In trials in ovarian and colorectal cancer, high-dose IV vitamin C was safe with chemotherapy and reduced side effects. It should not be given on the same day as bortezomib.

What kind of vitamin E is best?

Natural mixed tocopherols and tocotrienols. Synthetic alpha-tocopherol alone was linked to more second cancers in a Canadian radiation trial.

About the author. Dr. Silver, MD, graduated from Temple University School of Medicine, completed fellowship training in cardiology, and has practiced medicine for nearly fifty years. A lymphoma survivor of about twenty-five years, he is co-author, with Dr. Lindmeier, DHSc, of The New Reality of Cancer: The Science of Recovery. Read more about Dr. Silver, MD.

Research by Dr. Lindmeier, DHSc, cancer educator and co-author of The New Reality of Cancer.

This article is for general education only and is not individual medical advice. Always consult your own doctor before making any changes to your care.

© 2026 Lovely LYF LLC. All rights reserved. This article draws on the research in The New Reality of Cancer and may not be copied, republished or distributed without written permission.

For more in-depth information on this topic and many others, get our book, The New Reality of Cancer: The Science of Recovery.

Explore our guides: Stage 4 Cancer Options · Advanced Cancer Testing · Integrative Cancer Treatment Options · Natural Compounds and Cancer · Repurposed Drugs for Cancer · Cancer Treatment by Type

More articles on integrative cancer treatments and all articles.

References

1. Block KI, Koch AC, Mead MN, Tothy PK, Newman RA, Gyllenhaal C. Impact of antioxidant supplementation on chemotherapeutic efficacy: a systematic review of the evidence from randomized controlled trials. Cancer Treatment Reviews. 2007;33(5):407–418.
2. Ma Y, Chapman J, Levine M, Polireddy K, Drisko J, Chen Q. High-dose parenteral ascorbate enhanced chemosensitivity of ovarian cancer and reduced toxicity of chemotherapy. Science Translational Medicine. 2014;6(222):222ra18.
3. Wang F, He MM, Xiao J, et al. A randomized, open-label, multicenter, phase 3 study of high-dose vitamin C plus FOLFOX ± bevacizumab versus FOLFOX ± bevacizumab in unresectable untreated metastatic colorectal cancer (VITALITY study). Clinical Cancer Research. 2022;28(19):4232–4239.
4. Ambrosone CB, Zirpoli GR, Hutson AD, et al. Dietary supplement use during chemotherapy and survival outcomes of patients with breast cancer enrolled in a cooperative group clinical trial (SWOG S0221). Journal of Clinical Oncology. 2020;38(8):804–814.
5. Bairati I, Meyer F, Gélinas M, et al. A randomized trial of antioxidant vitamins to prevent second primary cancers in head and neck cancer patients. Journal of the National Cancer Institute. 2005;97(7):481–488.
6. Sayin VI, Ibrahim MX, Larsson E, Nilsson JA, Lindahl P, Bergo MO. Antioxidants accelerate lung cancer progression in mice. Science Translational Medicine. 2014;6(221):221ra15.
7. Le Gal K, Ibrahim MX, Wiel C, et al. Antioxidants can increase melanoma metastasis in mice. Science Translational Medicine. 2015;7(308):308re8.

This article is for education only. It is not medical advice and does not diagnose or treat any condition. Cancer Expert MD has no financial relationship with any laboratory, drug maker, supplement company, clinic or hospital.