The short answer
"Natural" is a marketing word, not a safety rating. Herbs interact with cancer drugs, supplements are not reviewed before sale, and using alternative medicine instead of treatment worsens survival.
Rating: not supported. "Natural" describes where something came from, not whether it is safe or whether it works.
St John's wort induces the CYP3A4 enzyme; in a crossover study, it cut levels of irinotecan's active metabolite by 42%.
FDA does not approve dietary supplements before they are sold — manufacturers, not regulators, vouch for safety.
In a 2018 JNCI study, people who used alternative medicine alone for curable cancers had 2.5 times the risk of death; five-year survival was 54.7% versus 78.3%.
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The full explanation.
The claim
Plant-based, natural and traditional remedies are gentler and safer than chemotherapy, radiation or surgery. So a natural route is the lower-risk choice.
Why it appeals
Because conventional treatment sounds frightening, and a herb sounds like food. Because a natural therapy is something you can choose and control, at a moment when very little feels controllable. Because it is often cheaper, or seems to be. Because someone kind spent an hour listening to you, and a clinic gave you fifteen minutes. And because plenty of people have good reasons not to hand medicine automatic trust — cost, past mistreatment, faith, a bad experience. None of that is foolish.
What the evidence shows
"Natural" is not a safety category. Foxglove, hemlock and tobacco are natural. What matters is dose, purity, and what else is in your body at the time.
Herbs can weaken cancer drugs. St John's wort is the clearest example. It induces the liver enzyme CYP3A4, which many cancer treatments depend on. A crossover study in patients receiving irinotecan was published in the Journal of the National Cancer Institute in 2002. Taking St John's wort cut plasma levels of the active metabolite SN-38 by 42%. That is not a side effect. That is less of the drug reaching the cancer. NCCIH lists St John's wort as weakening some cancer medications, including irinotecan. It does the same to warfarin, birth control and HIV drugs.
Supplements are not checked before sale. FDA states it does not have the authority to approve dietary supplements before they are marketed. Manufacturers are responsible for their own products' safety. FDA acts after something goes wrong. Contamination, mislabelled doses and hidden pharmaceutical ingredients turn up regularly in enforcement.
Used instead of treatment, the outcomes are worse. Researchers at Yale used the National Cancer Database. They identified 281 people with non-metastatic breast, prostate, lung or colorectal cancer who chose alternative medicine as their only cancer treatment. Each was matched 2:1 against people who had conventional treatment. The results were published in JNCI in 2018. The alternative-medicine group had 2.5 times the risk of death overall (hazard ratio 2.50). Five-year survival was 54.7% versus 78.3%. For breast cancer the gap was widest: 58.1% versus 86.6%.
A companion study in JAMA Oncology the same year looked at people who used complementary medicine alongside treatment. They also died sooner. But the analysis showed why. They refused surgery, chemotherapy, radiation or hormone therapy at much higher rates. The complementary therapy itself did not explain it.
NCCIH puts the summary plainly. No complementary health approach has been shown to prevent or cure cancer.
What this does not mean
It does not mean everything outside a hospital is useless. Acupuncture for nausea, exercise for fatigue, mindfulness for anxiety and massage for pain are offered inside major cancer centers. There is evidence behind them for symptoms. It does not mean your instinct to look for something more is a character flaw. And it does not mean that if you have already tried something, you have ruined anything.
The line that matters is between alongside and instead of.
The bottom line
"Natural" tells you where something came from. It does not tell you whether it is safe, or whether it works. The specific risks are interactions with your treatment, and products no one checked before selling. The largest risk is delaying or replacing a treatment that could have cured you.
The most useful thing you can do is simple. Bring the actual bottle, or the practitioner's name, to your oncology team, and ask directly. Teams are used to this question. Being told what you are taking helps them far more than being told nothing.
Sources
- Johnson et al., JNCI 2018 — Use of alternative medicine for cancer and its impact on survival
- Johnson et al., JAMA Oncology 2018 — Complementary medicine, refusal of conventional cancer therapy, and survival
- Mathijssen et al., JNCI 2002 — Effects of St John's wort on irinotecan metabolism
- NCCIH — St. John's Wort
- FDA — Questions and Answers on Dietary Supplements
- NCI — Complementary and Alternative Medicine
Words to know
Tap any term to see what it means.
Common questions
What is the difference between complementary and alternative?
Complementary means used alongside standard treatment, usually for symptoms and quality of life. Alternative means used instead of it. The evidence on harm is overwhelmingly about the second.
Can herbs really interfere with chemotherapy?
Yes. St John's wort induces the liver enzyme CYP3A4, which many cancer drugs depend on. In a crossover study in patients, it lowered levels of irinotecan's active metabolite by 42% — meaning less drug reaching the cancer. NCCIH lists it as weakening some cancer medications.
Are supplements checked for safety before they are sold?
Not in the way medicines are. FDA states it does not have the authority to approve dietary supplements before they are marketed. The manufacturer is responsible for safety, and FDA acts after a problem appears. Contamination and undisclosed ingredients turn up in enforcement actions.
Does that mean acupuncture, yoga and herbal tea are all off-limits?
No. Many supportive approaches are offered inside cancer centers for nausea, pain, sleep and anxiety. The question is whether something replaces treatment or interacts with it. That is answerable — bring the actual bottle or the practitioner's name to your team.
My reasons for distrusting conventional medicine are personal. Does that count?
It does, and it is worth saying out loud to your team. Cost, past mistreatment, faith, and bad experiences are real reasons people look elsewhere. None of them change what a herb does to a drug level — but they do change what a good conversation with your team needs to cover.
Questions to ask your doctor
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Written by: Cancer ExplainedSources last checked: 2026-08-13 what this meansLast updated: 2026-08-18Next planned review: 2027-01-30
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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. 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.
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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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