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, antioxidant supplements during chemotherapy or radiation have been linked to worse outcomes in some studies, grapefruit changes the levels of some drugs, and green tea extract in pill form has rarely damaged the liver. 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.
Antioxidant supplements taken during chemotherapy or radiation have been linked to a higher risk of death and a lower chance of staying cancer free in some studies.
Grapefruit and grapefruit juice can raise or lower levels of certain cancer drugs unpredictably, depending on the drug.
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"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. Some people take them hoping to protect healthy cells 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. In some head and neck cancer trials, vitamin E supplements were linked to a higher risk of the cancer coming back. The NCI says more research is needed here. This concern is about supplements. Ask your oncologist before taking antioxidant supplements during active treatment.
Green tea extract
NCI says risks and side effects come from drinking large amounts of green tea, more than 8 cups a day. It also says that in rare cases, green tea extracts taken in pill form have caused liver damage. The FDA's Division of Drug Oncology Products recommends that trial participants take the extract with food. It also recommends 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. The NCI summary this page relies on does not cover them. Do not assume a supplement is neutral just because little is known about it. Ask your pharmacist.
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. Do not drive yourself.
- 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 herbal pill. Yellow skin or eyes and dark urine are signs of jaundice (MedlinePlus). Do not take another dose. Call the team first.
- 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 your team may want to review the product before your next cycle.
Sources
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?
NCI says people who took it during treatment had lower levels of anticancer drugs in their blood, so those drugs may work less well. 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. NCI describes studies that linked antioxidant supplements taken during chemotherapy and radiation to a higher risk of death and a lower chance of staying cancer free. NCI says more research is needed. The concern is about supplements. Ask your care team what to eat and take during treatment.
Can I still drink coffee or tea if green tea extract is risky?
NCI says that in rare cases, green tea extracts in pill form have caused liver damage. It also says risks and side effects come from drinking large amounts of green tea, more than 8 cups a day. NCI says studies have not shown whether green tea is safe with anticancer drugs. Mention any green tea you drink often, or any extract, to your care team.
Why does grapefruit matter if I'm not taking a supplement?
NCI says grapefruit juice, and a compound in grapefruit seeds, are known to interact with some anticancer drugs. In studies, grapefruit juice lowered the amount of some drugs that reached the blood and raised it for others. Because the effect depends on the drug, ask your pharmacist whether grapefruit is a concern for your medicines.
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Last updated: 2026-09-29Next planned review: 2027-08-03
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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, and this 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 then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. 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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