The short answer
Herbal supplements are not automatically safe just because they're natural. St. John's wort can make some cancer drugs less effective, high-dose antioxidants during radiation may reduce its effectiveness, grapefruit changes drug levels unpredictably, and green tea extract in pill form has been linked to liver injury. Tell your pharmacist about everything you take.
'Natural' does not mean risk-free — herbal supplements can change how cancer drugs work in the body.
St. John's wort can lower the blood levels of some anticancer drugs, potentially making them less effective.
High-dose antioxidant supplements taken during radiation or chemotherapy have been linked to worse outcomes in some studies, possibly by protecting cancer cells along with healthy ones.
Grapefruit and grapefruit juice can raise or lower levels of certain cancer drugs unpredictably, depending on the drug.
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The full explanation.
"Natural" does not mean risk-free
Herbs, plant extracts, and high-dose vitamins are often sold as gentle. They come from plants, after all. But their active compounds are still chemicals. They can change how your body handles other drugs, cancer drugs included. Some change how fast your body clears a drug. Others affect how well it reaches the cancer. "Natural" says nothing about whether a substance is safe with chemotherapy, targeted therapy, or radiation.
St. John's wort
St. John's wort is often used for low mood. Studies found that people who took it with anticancer drugs had lower drug levels in their blood. A drug may work less well when its level drops. St. John's wort also interacts with medicines for depression. It can push serotonin to unsafe levels. Tell your oncology team if you take it, or plan to.
High-dose antioxidants during radiation or chemotherapy
Antioxidants are sold for general health. So it seems logical that they would protect the body during treatment. In some cases research found the opposite. One study followed postmenopausal breast cancer survivors. It found a higher risk of death, and a lower chance of staying cancer free, when antioxidant supplements were used during chemotherapy and radiation. Some head and neck cancer trials also found a higher risk of the cancer coming back. The NCI says more research is needed here. This concern is about concentrated supplement doses. It is not about antioxidant-rich foods such as fruit and vegetables, which belong in a healthy diet during treatment. Ask your oncologist before taking antioxidant supplements during active treatment.
Green tea extract
Brewed green tea is one thing. Concentrated green tea extract, sold in pills or capsules, is another. In rare cases that extract has damaged the liver. The FDA's Division of Drug Oncology Products advises trial participants to take it with food. It also advises that liver function tests be considered during treatment. There is a case report of a man with metastatic kidney cancer. His symptom control got worse while he drank green tea, and improved once he stopped. Mention any concentrated green tea product to your care team.
Grapefruit
Grapefruit and grapefruit juice can change how much of a drug reaches your blood. The direction is not always the same. One small trial found that less of the anticancer drug etoposide got through in people who drank grapefruit juice. Studies of imatinib, sunitinib, and nilotinib found the opposite. There, grapefruit juice raised the amount that reached the blood. The effect depends on the drug. Ask your pharmacist whether grapefruit matters for your exact medicines.
Turmeric and other supplements
Turmeric, curcumin, and many other herbal products are widely used and sold for wellness. Solid evidence about their effect on specific cancer drugs is often limited or mixed. Do not assume a supplement is neutral just because little is known about it.
The one rule that covers all of this
You do not need to memorize every possible interaction. What matters is telling your oncology pharmacist about everything you take. That means every herb, vitamin, supplement, and over-the-counter product, not only prescriptions. Bring the bottles or a written list to your next visit, and ask before starting anything new.
When to get help sooner
- Call 911 or go to an emergency department if you take St. John's wort alongside an antidepressant and become agitated or confused, with a racing heart, twitching or jerky muscles, shivering, diarrhea and a rising body temperature. That cluster can be serotonin syndrome, and MedlinePlus notes it can begin within minutes to hours of a dose.
- Call your care team the same day if the whites of your eyes or your skin turn yellow, your urine goes dark, or you feel sick and sore under the right ribs while taking green tea extract or any concentrated herbal pill. LiverTox reports these as the usual first signs of extract-related liver injury. Stop the product and ring the team before your next dose.
- Call your care team within a day or two if you realise you have been taking a herb, high-dose antioxidant or other supplement through active treatment without telling anyone. Nothing may be wrong, but the pharmacist may want a blood level checked or the product stopped before your next cycle.
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Common questions
Why would a natural supplement interfere with cancer treatment?
Herbs and plant extracts contain active chemical compounds, just like medicines do. Some of those compounds affect the same liver enzymes or processes that break down cancer drugs, which can raise or lower the amount of drug that stays active in your body. 'Natural' describes the source, not the safety.
Is St. John's wort dangerous during cancer treatment?
It can reduce the blood levels of several anticancer drugs by speeding up how fast the liver clears them, which may make those drugs less effective. It can also interact dangerously with certain antidepressants. If you take St. John's wort, tell your oncology team before starting or continuing any cancer treatment.
Should I stop taking antioxidant vitamins during radiation?
Ask your radiation oncologist first. Research has linked high-dose antioxidant supplements taken during radiation or chemotherapy to a higher risk of the cancer coming back and lower survival in some studies, possibly because antioxidants can protect cancer cells from treatment as well as normal cells. This is different from eating antioxidant-rich foods, which is generally considered fine — the concern is specifically about concentrated supplement doses.
Can I still drink coffee or tea if green tea extract is risky?
The liver concern is mainly linked to concentrated green tea extract in pill or capsule form, taken in amounts far higher than what's in a cup of brewed tea. Ordinary brewed green tea is a different situation, but if you drink large amounts or take any concentrated extract, mention it to your care team.
Why does grapefruit matter if I'm not taking a supplement?
Grapefruit and grapefruit juice contain natural compounds that block an enzyme many drugs rely on for normal breakdown. This can cause some drugs to build up to unexpectedly high levels, and can lower others. Because the effect depends on the specific drug, ask your pharmacist whether grapefruit is a concern for your particular medicines.
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Last updated: 2026-08-11Next planned review: 2027-08-03
How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status — Source checked. This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
High-risk topic — talk to your care team. This topic can involve urgent, individual medical decisions. This page is general education only: it cannot tell you whether your situation is an emergency or what you personally should do. Follow your oncology team's instructions and contact them for individual guidance.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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