Record ID: MS-N070 · Family: Dietary supplements / treatment compatibility / breast cancer · Clinical question: Are antioxidant supplements taken during adjuvant chemotherapy associated with recurrence or survival?

Evidence label: Concerning but noncausal observational signals; randomized evidence for cancer outcomes is absent.

Evidence at a glance

• Benefit: No reliable survival or recurrence benefit has been demonstrated.
• Certainty: Low. The key analyses are observational, even when nested within a randomized chemotherapy trial.
• Risk: Several cohorts reported worse outcomes with concurrent antioxidant use, but confounding and exposure misclassification remain plausible.
• Cancer control: Associations concern recurrence and mortality; they do not prove that supplements caused those outcomes.

What was studied

The DELCaP cohort followed 1,134 people enrolled in SWOG S0221, a randomized adjuvant chemotherapy trial for high-risk early breast cancer. Supplement use was self-reported before treatment and again during chemotherapy. The supplement comparison itself was not randomized.

A separate German MARIE cohort included 2,223 postmenopausal women with nonmetastatic breast cancer; 1,940 contributed to the concurrent-treatment analysis. A later systematic review combined heterogeneous post-diagnosis antioxidant studies.

What the results show

In DELCaP, taking any antioxidant supplement—vitamins A, C or E, carotenoids, or coenzyme Q10—both before and during chemotherapy was associated with a higher recurrence hazard (adjusted HR 1.41, 95% CI 0.98–2.04; P=.06) and death hazard (adjusted HR 1.40, 95% CI 0.90–2.18; P=.14). Both confidence intervals included no association.

In MARIE, antioxidant use concurrent with chemotherapy or radiotherapy was associated with total mortality (HR 1.64, 95% CI 1.01–2.66) and worse recurrence-free survival (HR 1.84, 95% CI 1.26–2.68). By contrast, a 2021 meta-analysis found no statistically significant overall-survival association during chemotherapy (HR 1.15, 95% CI 0.78–1.68).

Important limits

• Supplement users can differ from nonusers in health, diet, treatment adherence and reasons for supplement use; adjustment cannot remove all confounding.
• Products, doses and timing varied, and food sources of antioxidants were not the exposure being tested.
• DELCaP was ancillary to a randomized treatment trial, but supplement exposure was observational.
• Funding and conflict details are available in the original reports; no product-specific sponsor influence was verified for this record.

Safety and practical context

The signal is strong enough to justify caution, not strong enough to claim proven harm. During chemotherapy, review every antioxidant product and dose with the oncology pharmacist. Correcting a documented deficiency is a separate clinical question from taking high-dose antioxidant products.

Questions for the care team

• What exact ingredients and doses are in the supplement?
• Is it being used to treat a verified deficiency or as a general antioxidant?
• Could it be paused during chemotherapy without creating another clinical risk?

Sources checked

• Ambrosone et al., Journal of Clinical Oncology 2020 — DELCaP full text (PMID 31855498; PMCID PMC7062457).
• Jung et al., American Journal of Clinical Nutrition 2019 — MARIE cohort (PMID 30668630).
• Li et al., Breast 2021 — systematic review and meta-analysis (PMID 34635464).

Last literature search: October 19, 2026 · Record appraisal: October 19, 2026

Educational information only. This record is not a prescription and does not replace advice from the oncology team or oncology pharmacist.