High-Dose Vitamin C in Breast Cancer Patients Undergoing Chemotherapy

Guideline-Based Recommendations

The American Cancer Society explicitly advises against using very large doses of vitamins during cancer treatment, due to the theoretical concern that antioxidants could repair oxidative damage to cancer cells caused by chemotherapy and radiotherapy, potentially reducing treatment efficacy 1, 2, 3
The 2020 ESO-ESMO International Consensus Guidelines for Advanced Breast Cancer specifically list "high-dose vitamins (vitamin C, D, E, carotenoids, etc.)" as not recommended in advanced breast cancer 4
During chemotherapy or radiotherapy, it is prudent to avoid exceeding the tolerable upper limits of the Dietary Reference Intakes for antioxidant vitamins such as vitamins C and E, as they could simultaneously protect cancer cells from treatment-induced damage 3

The potential harm from antioxidants during treatment remains largely hypothetical, and some experts note there may be net benefit in protecting normal cells from collateral damage, representing an ongoing scientific debate without clear resolution 3
A standard multivitamin containing approximately 100% of the Daily Value is considered acceptable and potentially beneficial during treatment, as it may be difficult to maintain adequate micronutrient intake through diet alone during this period 1, 2
The use of high-dose supplements (much higher than 100% Daily Value) is where caution is specifically advised, with the distinction between physiologic doses and pharmacologic doses being critical 1, 5

Between 31% to 68% of cancer patients using supplements do not discuss this with their physicians, which is particularly concerning given potential drug interactions 6
Some supplements can directly interfere with chemotherapy efficacy, such as folic acid supplementation being counterproductive when receiving methotrexate, which acts by interfering with folic acid metabolism 3

Avoid high-dose vitamin C supplements (>100% Daily Value) during active chemotherapy based on guideline consensus from the American Cancer Society and the 2020 ESO-ESMO International Consensus Guidelines for Advanced Breast Cancer 1, 3, 4
A standard multivitamin (approximately 100% Daily Value) is acceptable if dietary intake is inadequate, according to the American Cancer Society 1, 2, 5
All supplement use must be disclosed to the oncology team to screen for potential drug interactions, as recommended by the American Cancer Society and the Journal of Clinical Oncology 1, 6