# Dietary supplements during cancer treatment: interactions and harms

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

## TL;DR

Most people on cancer treatment take supplements, often without telling their team. Antioxidants, St John's wort, high-dose vitamins and some herbs can blunt chemotherapy or radiotherapy or interact with targeted drugs.

## Summary

Surveys find 60-80% of patients use dietary supplements during treatment and roughly half do not disclose it. Evidence of harm is strongest for: antioxidant supplements (vitamins A, C, E, carotenoids, coenzyme Q10) during chemotherapy and radiotherapy, associated with higher recurrence and death in the DELCaP cohort within a breast cancer chemotherapy trial (JCO 2020: recurrence HR 1.41) and with worse outcomes in head and neck cancer radiotherapy (randomised, beta-carotene plus vitamin E); beta-carotene increasing lung cancer in smokers (ATBC, CARET randomised trials); vitamin E and selenium increasing prostate cancer in SELECT; St John's wort inducing CYP3A4 and reducing exposure to irinotecan, imatinib and many tyrosine kinase inhibitors; and high-dose vitamin B12 and B6 associated with lung cancer in men. Iron and vitamin B12 during checkpoint or targeted therapy are usually harmless; green tea extract and kava carry hepatotoxicity risk that confounds drug-induced liver injury assessment. Memorial Sloan Kettering's About Herbs database and pharmacist-led medication reconciliation that includes supplements are the practical tools.

## Fields

- Kind: Technology
- Status: established
- Last checked: 2026-09-08
- Principle: Antioxidants can neutralise the reactive oxygen species through which radiotherapy and many cytotoxics kill cells; herbal constituents induce or inhibit CYP450 enzymes and drug transporters, altering the exposure of oral anticancer drugs.
- Strengths: Randomised evidence of harm for several supplements; Interaction databases and pharmacist review are cheap fixes; Disclosure improves with routine, non-judgemental asking
- Limitations: Most supplement evidence is observational; Products vary in content and contamination; Regulatory oversight is weak in most countries

## Sources

- Wikipedia: https://en.wikipedia.org/wiki/Dietary_supplement
- DELCaP: supplements during chemotherapy (JCO 2020): https://doi.org/10.1200/JCO.19.01203
- MSK About Herbs database: https://www.mskcc.org/cancer-care/diagnosis-treatment/symptom-management/integrative-medicine/herbs

## Connected records

- ideas: [An open supplement-drug interaction checker built into oncology prescribing](https://onco.cc/ideas/idea-moon-supplement-interaction-database/), [Ask about diet at diagnosis, and prebunk the myths before the internet does](https://onco.cc/ideas/idea-nl-ask-about-diet-prebunking/), [Confirm or refute the harm of antioxidant supplements during chemotherapy](https://onco.cc/ideas/idea-nl-antioxidant-supplement-harm-confirmation/)
- cancers: [Colorectal cancer](https://onco.cc/cancers/colorectal/), [Head and neck squamous cell carcinoma](https://onco.cc/cancers/head-and-neck/), [HR-positive / HER2-negative breast cancer](https://onco.cc/cancers/breast-hr-positive/), [Non-small-cell lung cancer](https://onco.cc/cancers/nsclc/), [Prostate cancer](https://onco.cc/cancers/prostate/), [Triple-negative breast cancer (TNBC)](https://onco.cc/cancers/tnbc/)
- fronts: [Diet, Exercise & Lifestyle](https://onco.cc/fronts/nutrition-lifestyle/), [Supportive Care & Survivorship](https://onco.cc/fronts/supportive-care/)
- technologies: [American ginseng for cancer-related fatigue](https://onco.cc/technologies/american-ginseng-fatigue/), [Ayurvedic medicine and cancer](https://onco.cc/technologies/ayurvedic-medicine-cancer/), [Cannabis and cannabinoids for pain, appetite and cancer control](https://onco.cc/technologies/cannabinoids-pain-appetite/), [Curcumin and turmeric](https://onco.cc/technologies/curcumin-turmeric/), [Cytotoxic chemotherapy](https://onco.cc/technologies/cytotoxic-chemotherapy/), [Green tea and EGCG extracts](https://onco.cc/technologies/green-tea-egcg/), [Medicinal mushrooms: reishi, turkey tail, shiitake and others](https://onco.cc/technologies/medicinal-mushrooms-reishi-turkey-tail/), [Melatonin as a cancer adjunct](https://onco.cc/technologies/melatonin-cancer/), [Oncology nutrition assessment and medical nutrition therapy](https://onco.cc/technologies/nutrition-screening-mnt/), [Soy foods and breast cancer](https://onco.cc/technologies/soy-breast-cancer/), [St John's wort: an interaction to avoid](https://onco.cc/technologies/st-johns-wort-interaction/), [Supplements sold for hair growth after cancer treatment](https://onco.cc/technologies/hair-supplements-marketed/), [Traditional Chinese herbal medicine alongside treatment](https://onco.cc/technologies/traditional-chinese-herbal-medicine/)
- drugs: [Doxorubicin](https://onco.cc/drugs/doxorubicin/), [Irinotecan (and liposomal irinotecan)](https://onco.cc/drugs/irinotecan/), [Paclitaxel / nab-paclitaxel](https://onco.cc/drugs/paclitaxel/)
- institutions: [Memorial Sloan Kettering Cancer Center](https://onco.cc/institutions/mskcc/)
- terms: [Unproven diet claims (alkaline, juice, 'anti-cancer' diets)](https://onco.cc/terms/unproven-diet-claims/)
- bottlenecks: [Misinformation and unproven therapies](https://onco.cc/bottlenecks/b-misinformation/), [Toxicity and quality of life are undervalued](https://onco.cc/bottlenecks/b-toxicity-qol/)
- key papers: [Dietary Supplement Use During Chemotherapy and Survival Outcomes of Patients With Breast Cancer Enrolled in a Cooperative Group Clinical Trial (SWOG S0221)](https://onco.cc/key-papers/paper-ambrosone-j-clin-oncol/)
- roadmaps: [Diet, exercise and lifestyle roadmap: causes established → interventions that disappointed → exercise proven as treatment](https://onco.cc/roadmaps/nutrition-lifestyle-roadmap/)

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