Supplements During Cancer Treatment: Safety and Evidence

Supplements are not automatically safe just because they are sold without a prescription. During cancer treatment, a few of them can genuinely interfere with the treatment that is working to save your life. This article names the specific ones that come up most often, explains why, and tells you what to do about it.

A supplement can affect bleeding, anesthesia, liver enzymes, kidney function, immune signaling, hormone pathways, and — most importantly — how your body absorbs, activates, and clears cancer drugs. The mechanism that makes an herb pharmacologically interesting is the same mechanism that makes it interact.

St John's wort. This is the clearest one. It strongly induces the CYP3A4 enzyme system and P-glycoprotein, which lowers blood levels of many cancer drugs. In a clinical study, St John's wort substantially reduced levels of the active metabolite of irinotecan (Mathijssen, J Natl Cancer Inst 2002). It affects many targeted therapies the same way. This can mean receiving less treatment than your oncologist prescribed, without anyone knowing. Do not take it during cancer treatment.

High-dose antioxidants during radiation or certain chemotherapy. Radiation and some chemotherapy drugs work partly by generating oxidative stress inside cancer cells. High-dose antioxidant supplements — vitamin C, vitamin E, NAC, alpha-lipoic acid, glutathione — could in principle blunt that. A randomized trial in head and neck cancer found worse outcomes in patients taking high-dose antioxidants during radiotherapy (Bairati, J Clin Oncol 2005), and a large observational analysis within a breast cancer trial found antioxidant supplement use during chemotherapy associated with worse outcomes (Ambrosone, J Clin Oncol 2019).

The evidence is not complete, and we want to be honest about that. But the reasoning we apply is about asymmetry: the cost of skipping a supplement for a few weeks is essentially nothing, and the cost of interfering with treatment given to cure you is enormous. Our default is dietary-level antioxidants only during active treatment, unless your oncologist agrees otherwise. Fruits and vegetables are a different matter and are encouraged.

Anything that thins the blood, before surgery or with a low platelet count. High-dose fish oil, vitamin E, ginkgo, garlic concentrate, high-dose curcumin, and others. Your surgical team will usually want these stopped one to two weeks before a procedure.

Probiotics, if you are immunocompromised. Bloodstream infection from a probiotic is uncommon — one hospital review found 8 cases across roughly 128,000 admissions over 11 years — but nearly all occurred in people with a central line, impaired intestinal function, or a weakened immune system (Hefter, Hosp Pediatr 2023). Those describe many people in cancer treatment. Separately, a study in melanoma found that people taking commercial probiotics had lower gut bacterial diversity and a less favorable response to immunotherapy, while those eating more dietary fiber did better (Spencer, Science 2021).

"Immune-boosting" supplements during immunotherapy. Medicinal mushroom extracts, echinacea, astragalus, and similar products. There is no evidence any of them improves how immunotherapy works, and there is a theoretical concern about worsening immune-related side effects — which in immunotherapy can be serious. If a reaction does occur, it is treated by suppressing the immune system, and an undisclosed stimulant complicates that.

High-dose vitamin B6. Above roughly 50–100 mg daily long-term, B6 causes nerve damage — a real problem when platinum drugs, taxanes, vincristine, or bortezomib are already causing neuropathy.

Zinc, long-term, without checking copper. Above about 40 mg a day, zinc depletes copper, which can cause low blood counts and neurological problems that look like other serious conditions.

Folate, if you are on methotrexate or pemetrexed. These drugs work through the folate pathway deliberately. Folate supplementation is part of the protocol for some and is carefully specified. Never change it on your own.

Some antidepressants with tamoxifen. Not a supplement, but it comes up: strong CYP2D6 inhibitors — paroxetine, fluoxetine, bupropion — reduce activation of tamoxifen. Worth raising with whoever prescribes them.

Bring every label, or photographs of them. Dose, frequency, how long you have been taking it, and why. Include herbs, mushrooms, teas, powders, vitamins, minerals, cannabis products, enzymes, and over-the-counter medications. "A multivitamin" is often a fifteen-ingredient proprietary blend, and we cannot check what we cannot read.

Use supplements to correct a documented deficiency or to address a specific symptom, where the benefit, timing, and safety are clear. Avoid high-dose, multi-agent protocols during active treatment unless your oncology team has reviewed them.

A useful question for anything you take: "what is this for, and how would we know if it worked?" Anything without an answer is worth reconsidering.

We review everything you take against your specific treatment, and we give you a current one-page list to hand your oncology team. Patients tell us it is the most useful thing we produce. Sometimes the most valuable outcome of a visit is stopping something — that is a real clinical act, and it reduces risk.

SANAVITA Health provides physician-led integrative oncology education and support with a focus on clarity, safety, whole-person care, and collaboration. The goal is to help patients understand their options, reduce avoidable risk, and build a supportive plan that fits their diagnosis, treatment phase, values, and care team recommendations.