Dietary supplements during cancer treatment: interactions and harms
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.
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.
How it works
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.
- Randomised evidence of harm for several supplements
- Interaction databases and pharmacist review are cheap fixes
- Disclosure improves with routine, non-judgemental asking
- Most supplement evidence is observational
- Products vary in content and contamination
- Regulatory oversight is weak in most countries
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