Supplements and Cancer Treatment: Chemotherapy, Radiation and Targeted Drugs

Safety box: tell the oncology team about everything

Cancer treatment is a time to be careful with supplements, not a time to guess. The US National Cancer Institute (NCI) says: "Tell your doctor if you're taking any dietary supplements, even vitamins, no matter how safe you think they are." [1] NCCIH says people diagnosed with cancer should consult their health care providers before using any complementary health approach. [2] A natural product is never a substitute for cancer treatment; NCCIH cites guidelines warning that delayed treatment of cancer reduces the likelihood of a remission or cure. [2] If you are in emotional crisis in the US, call or text 988. For collapse or trouble breathing, call 911.

The question

Can supplements interfere with chemotherapy, radiation or targeted drugs, and what is actually known? This page separates what is documented, what is observational, and what is theoretical. It does not say whether any supplement helps with cancer.

General statement from NCCIH

NCCIH says some complementary approaches can interfere with medical treatment for cancer, for example "some herbs may make chemotherapy drugs less effective." [2]

Antioxidant supplements (vitamins A, C and E, carotenoids, coenzyme Q10)

• Concern. NCI explains that ionizing radiation kills cells by producing free radicals, and that antioxidants neutralize free radicals. [3] Whether antioxidant supplements could blunt radiation or some drugs is therefore a reasonable question, but it is a theoretical link; the sources we read do not include a radiation-specific trial. Label: theoretical.
• Randomized trials. NCI says several small randomized trials of antioxidant supplements during cancer treatment had mixed results, but "some found that people who took antioxidant supplements during cancer therapy had worse outcomes, especially if they were smokers." [3] It advises caution until more is known. [3] Label: mixed, with some evidence of harm.
• Animal studies. In some preclinical studies, antioxidants promoted tumor growth and spread in mice. [3] Label: preclinical; not proof for people.

A large observational study in breast cancer (SWOG S0221)

Researchers asked 1,134 patients with breast cancer in a chemotherapy trial about supplement use and followed their outcomes. [4] Findings, as reported in the 2020 paper:

• Any antioxidant supplement before and during treatment was associated with a higher risk of recurrence, but the result was borderline and not statistically conclusive (adjusted hazard ratio 1.41, 95% CI 0.98 to 2.04, P = .06). [4]
• Vitamin B12 use before and during chemotherapy was significantly associated with poorer disease-free survival. [4]
• Iron use during chemotherapy was significantly associated with recurrence. [4]
• Multivitamin use was not associated with survival outcomes. [4]

The authors say the results are consistent with caution about supplements other than a multivitamin during chemotherapy. [4] This was an observational study (it shows association, not cause), done in one cancer type and one chemotherapy regimen. Label: promising signal of harm, limited evidence. See also Vitamins During Cancer Treatment and Antioxidant Safety and Marketing Claims.

St. John's wort and cancer medicines

NCCIH lists irinotecan, imatinib and docetaxel among cancer medicines St. John's wort can affect. [5] NCI says St. John's wort may cause certain cancer drugs not to work as well as they should. [1] A 2009 review lists reduced levels of anticancer drugs such as irinotecan and imatinib among documented interactions. [6] Label: documented. Imatinib is a targeted drug; the other sources we read do not discuss targeted drugs in general. Details: St. John's Wort Interactions.

Not covered

Proper evidence not found, in the sources read for this page, for most individual herbs and chemotherapy or targeted-drug interactions. Absence of evidence is not safety. Chinese and Ayurvedic herbal products are covered on Chinese Herbs During Cancer Treatment.

What a reader can do

• Bring the actual bottles or a photo of the labels to oncology visits, including teas and "immune support" products.
• Ask the oncologist and an oncology pharmacist before starting or stopping anything. Do not change a cancer medicine because of a supplement.
• Read Integrative Care During Cancer Treatment and Surgery and Procedures if surgery is planned.
• Be wary of any product promising a cancer cure; NCCIH notes some such products are dangerous in themselves or harmful if used instead of treatment. [2]